The PSTN and a PBX operate at different scales. The public switched telephone network (PSTN) is the shared, carrier-operated system of interconnected telephone networks that can reach almost any phone in the world,[4][5] while a private branch exchange (PBX) is a single organization's own telephone switch that routes calls between its internal users and shares a small number of outside lines among them.[1] The PSTN belongs to national, regional, and local operators and works as public infrastructure,[5] whereas a PBX is privately owned and usually sits on a company's premises or, increasingly, in a provider's cloud.[1][2] The two are not competitors but layers that connect: a PBX links to the PSTN through trunk lines so its extensions can call the outside world, which is why an office needs far fewer external lines than it has phones.[1][3] Modern IP PBX and hosted PBX systems perform the same switching in software and often reach the PSTN over a SIP trunk instead of copper.[2][10]
\n","summary":"The PSTN is the public, carrier-run network that connects phones worldwide; a PBX is a private switch that routes calls inside one organization and shares a few outside lines among many users. They are not rivals. A PBX plugs into the PSTN through trunk lines so staff can reach outside numbers, which is how a 100-person office runs on about ten external lines. Today most PBXs are IP or cloud-hosted and connect to the PSTN over a SIP trunk.","publishedAt":"2026-07-18T15:32:04.773","verifiedAt":"2026-07-18T00:00:00","editorialStatus":"APPROVED","lastReviewedAt":"2026-07-18T00:00:00","nextReviewDueAt":"2026-10-18T00:00:00","templateVersion":"v2","aliases":["PSTN vs PBX","PBX vs PSTN","What is the difference between a PBX and the PSTN?","How does a PBX connect to the PSTN?","Is a PBX the same as the PSTN?","Public switched telephone network vs private branch exchange","Difference between a business phone system and the public phone network","PBX and PSTN explained","How are the PSTN and a PBX related?","PSTN and PBX meaning","Do I need a PBX or just the PSTN?"],"confidenceScore":93,"confidenceLabel":"High","canonicalUrl":null},"contributor":{"id":"ec39deab-44fe-48d8-9029-fefe993ab85a","slug":"answer-stack","displayName":"AnswerStack","websiteUrl":null},"contributorOrganizationProfile":{"entityId":"ec39deab-44fe-48d8-9029-fefe993ab85a","legalName":null,"description":null,"websiteUrl":null,"imageUrl":null,"slogan":null,"subtitle":null,"facts":[],"coiNote":null,"foundingDate":null,"numberOfEmployeesText":null,"contactPoint":null,"address":null,"headquartersText":null,"organizationType":null},"contributorPerson":{"slug":"answerstack-editorial-team","displayName":"AnswerStack Editorial Team"},"sections":[{"id":"1ea707f8-fef4-4f8a-8070-85712de4adda","sectionKey":"how_they_differ","sectionType":"markdown_section","heading":"How do the PSTN and a PBX differ?","introMarkdown":"The clearest way to separate the two is by who each one serves. The PSTN, short for public switched telephone network, is the worldwide collection of interconnected telephone networks run by carriers, and its job is to connect any phone to any other phone across providers and countries.[4][5] A PBX, short for private branch exchange, is a switching system owned by a single organization that connects that organization's own phones to each other and gives them shared access to the outside world.[1][2] The PSTN is public infrastructure that everyone shares, while a PBX is private equipment that one business runs for its own staff, so they sit at very different scales even though they plug into each other.\n\nThe name of a PBX spells out the relationship. 'Private' means the system stays separate from the public network even though it connects to it, 'branch' refers to the way a smaller endpoint hangs off the main circuits, and 'exchange' describes the switching that lets many phones share a limited set of lines.[1] Before PBXs were common, a company that wanted ten desk phones to reach outside callers would have needed ten separate lines running to the telephone company's central office. A PBX removes that one-to-one requirement by concentrating traffic, so a business with 100 employees might run on roughly ten external lines, because most calls stay inside the building and the outside lines get used only when someone actually dials out.[2][3]\n\nThe two therefore solve different problems. The PSTN handles the routing that gets a call from one network to a phone on another, across regions and carriers, and a PBX handles the local routing inside one site plus the sharing of a handful of outside lines.[1][5] They meet at the edge of the premises, where trunk lines run from the PBX out to the PSTN, and beyond that boundary the public network takes over.[1][4] The sections that follow break the differences down one at a time, then explain how the two connect and where cloud systems are blurring the old line between them.","introHtml":"The clearest way to separate the two is by who each one serves. The PSTN, short for public switched telephone network, is the worldwide collection of interconnected telephone networks run by carriers, and its job is to connect any phone to any other phone across providers and countries.[4][5] A PBX, short for private branch exchange, is a switching system owned by a single organization that connects that organization's own phones to each other and gives them shared access to the outside world.[1][2] The PSTN is public infrastructure that everyone shares, while a PBX is private equipment that one business runs for its own staff, so they sit at very different scales even though they plug into each other.
\nThe name of a PBX spells out the relationship. 'Private' means the system stays separate from the public network even though it connects to it, 'branch' refers to the way a smaller endpoint hangs off the main circuits, and 'exchange' describes the switching that lets many phones share a limited set of lines.[1] Before PBXs were common, a company that wanted ten desk phones to reach outside callers would have needed ten separate lines running to the telephone company's central office. A PBX removes that one-to-one requirement by concentrating traffic, so a business with 100 employees might run on roughly ten external lines, because most calls stay inside the building and the outside lines get used only when someone actually dials out.[2][3]
\nThe two therefore solve different problems. The PSTN handles the routing that gets a call from one network to a phone on another, across regions and carriers, and a PBX handles the local routing inside one site plus the sharing of a handful of outside lines.[1][5] They meet at the edge of the premises, where trunk lines run from the PBX out to the PSTN, and beyond that boundary the public network takes over.[1][4] The sections that follow break the differences down one at a time, then explain how the two connect and where cloud systems are blurring the old line between them.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":0},{"id":"36cfb646-8d0b-4746-b62f-dda1d8d8f7ab","sectionKey":"comparison_table","sectionType":"table_section","heading":"PSTN vs PBX at a glance","introMarkdown":"The table sets the two side by side across the differences that matter most in practice. Each row has its own section below that explains why the difference matters and what to do with it.","introHtml":"The table sets the two side by side across the differences that matter most in practice. Each row has its own section below that explains why the difference matters and what to do with it.
\n","outroMarkdown":"None of these differences make one a replacement for the other, because a PBX only reaches outside callers by handing the call to the PSTN.[1]","outroHtml":"None of these differences make one a replacement for the other, because a PBX only reaches outside callers by handing the call to the PSTN.[1]
\n","contentJson":{"rows":[{"cells":["Ownership and reach","Public infrastructure owned by carriers; reaches phones across networks and countries [5]","Owned or subscribed by one organization; serves that organization's users [1]"]},{"cells":["Core function","Routes calls between exchanges, carriers, and regions [5]","Switches internal calls and shares a few outside lines [1]"]},{"cells":["Where it lives","In carrier exchanges and the long-haul network [5]","On the customer premises or in a provider's cloud [2]"]},{"cells":["Numbering","Full public numbers under ITU-T E.164 [8]","Short internal extensions, bridged to public numbers by DID [6]"]},{"cells":["Cost model","A carrier service, traditionally billed per line and per call [4]","Bought or subscribed, then billed only for shared trunks and calls [1]"]}],"columns":["Dimension","PSTN","PBX"]},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":1},{"id":"10bc34ef-726b-46ed-b16c-891ab733f7f7","sectionKey":"who_owns_each","sectionType":"markdown_section","heading":"Who owns each one, and how far it reaches","introMarkdown":"Ownership is the root difference, and it drives almost everything else. The PSTN is public infrastructure, built and run by national, regional, and local telephone operators that interconnect their networks so a call can cross from one carrier to another and reach a phone almost anywhere.[5] No single company owns it, and you connect to it as a subscriber by buying service from a carrier rather than by installing your own switch.[4] A PBX sits at the other end of the ownership scale: one organization owns or subscribes to it and runs it for its own people, which is what the word 'private' in private branch exchange means.[1] Historically that meant a switch in a back room that the company or its vendor maintained, and today it can also be software rented from a hosted provider.[2] The consequence is control. Because a business runs its own PBX, it decides the dial plan and how calls flow internally, along with which calling features to turn on, whereas the behavior of the public network is set by carriers and regulators.[1]","introHtml":"Ownership is the root difference, and it drives almost everything else. The PSTN is public infrastructure, built and run by national, regional, and local telephone operators that interconnect their networks so a call can cross from one carrier to another and reach a phone almost anywhere.[5] No single company owns it, and you connect to it as a subscriber by buying service from a carrier rather than by installing your own switch.[4] A PBX sits at the other end of the ownership scale: one organization owns or subscribes to it and runs it for its own people, which is what the word 'private' in private branch exchange means.[1] Historically that meant a switch in a back room that the company or its vendor maintained, and today it can also be software rented from a hosted provider.[2] The consequence is control. Because a business runs its own PBX, it decides the dial plan and how calls flow internally, along with which calling features to turn on, whereas the behavior of the public network is set by carriers and regulators.[1]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":2},{"id":"58d223b4-15c6-40c1-8dc9-2a08907d93d4","sectionKey":"what_each_switches","sectionType":"markdown_section","heading":"What each one actually switches","introMarkdown":"The two switch calls at very different scopes. A PBX switches calls between the phones at one location and connects those phones to a limited number of external lines, so an internal call from one desk to another never leaves the building or touches a carrier.[1] That local switching is where a PBX earns its keep, because internal calls cost nothing and only outbound calls use up a shared outside line.[3] The PSTN switches at a far larger scope, routing calls between exchanges, between carriers, and between countries through a hierarchy of switches so that any subscriber can reach any other.[5] A useful way to picture it is that a PBX is a small, private version of what a public telephone exchange does, scoped to one organization rather than a whole town or region.[1][5] When someone at a PBX phone dials an outside number, the PBX seizes an available trunk and hands the call to the PSTN, which carries out the wider routing to the destination.[1]","introHtml":"The two switch calls at very different scopes. A PBX switches calls between the phones at one location and connects those phones to a limited number of external lines, so an internal call from one desk to another never leaves the building or touches a carrier.[1] That local switching is where a PBX earns its keep, because internal calls cost nothing and only outbound calls use up a shared outside line.[3] The PSTN switches at a far larger scope, routing calls between exchanges, between carriers, and between countries through a hierarchy of switches so that any subscriber can reach any other.[5] A useful way to picture it is that a PBX is a small, private version of what a public telephone exchange does, scoped to one organization rather than a whole town or region.[1][5] When someone at a PBX phone dials an outside number, the PBX seizes an available trunk and hands the call to the PSTN, which carries out the wider routing to the destination.[1]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":3},{"id":"7b00fa82-3b11-4aec-8d20-eeeaded1a0ca","sectionKey":"where_it_lives","sectionType":"markdown_section","heading":"Where the equipment lives","introMarkdown":"Location is the difference most people can point to. A traditional PBX is physical equipment on the customer's premises, historically a cabinet where the incoming trunk lines terminate and connect to the internal phones.[1] The organization houses it, powers it, and maintains it, which is why an on-premises PBX is a capital purchase as much as a service.[2] The PSTN lives in the carrier's world instead: telephone exchanges, the trunks between them, and the long-haul network that spans regions, none of which sits inside the customer's building.[5] Hosted and cloud PBX systems have shifted this picture, because the switching software runs in a provider's data center and the business keeps only the phones or apps, paying a per-user subscription rather than buying hardware.[2] The switching logic stays the same; only its physical home changes, which is one reason the boundary between owning a PBX and buying a service from the network has softened.","introHtml":"Location is the difference most people can point to. A traditional PBX is physical equipment on the customer's premises, historically a cabinet where the incoming trunk lines terminate and connect to the internal phones.[1] The organization houses it, powers it, and maintains it, which is why an on-premises PBX is a capital purchase as much as a service.[2] The PSTN lives in the carrier's world instead: telephone exchanges, the trunks between them, and the long-haul network that spans regions, none of which sits inside the customer's building.[5] Hosted and cloud PBX systems have shifted this picture, because the switching software runs in a provider's data center and the business keeps only the phones or apps, paying a per-user subscription rather than buying hardware.[2] The switching logic stays the same; only its physical home changes, which is one reason the boundary between owning a PBX and buying a service from the network has softened.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":4},{"id":"af660a53-de93-4c72-8cc4-7b09df86f8c8","sectionKey":"numbering","sectionType":"markdown_section","heading":"How numbering works: public numbers vs extensions","introMarkdown":"The PSTN and a PBX identify phones with different kinds of numbers. Every phone reachable on the PSTN has a full public number under the international numbering plan defined in ITU-T Recommendation E.164, which gives each line a globally unique, routable address of up to 15 digits.[8] Inside a PBX, phones are usually reached by short internal extensions, often three or four digits, that only carry meaning within that system.[1] The two schemes are bridged by a carrier feature called direct inward dialing, or DID. With DID, the telephone company allocates a block of public numbers to the organization and delivers them over a smaller set of trunks, and the PBX maps each incoming public number straight to the right extension without an operator.[6] This is how a company can publish a distinct outside number for every employee while running on far fewer physical lines than it has numbers.[6] Internally, staff still dial the short extension; externally, callers reach the same person through the full E.164 number that DID points at that extension.[6][8]","introHtml":"The PSTN and a PBX identify phones with different kinds of numbers. Every phone reachable on the PSTN has a full public number under the international numbering plan defined in ITU-T Recommendation E.164, which gives each line a globally unique, routable address of up to 15 digits.[8] Inside a PBX, phones are usually reached by short internal extensions, often three or four digits, that only carry meaning within that system.[1] The two schemes are bridged by a carrier feature called direct inward dialing, or DID. With DID, the telephone company allocates a block of public numbers to the organization and delivers them over a smaller set of trunks, and the PBX maps each incoming public number straight to the right extension without an operator.[6] This is how a company can publish a distinct outside number for every employee while running on far fewer physical lines than it has numbers.[6] Internally, staff still dial the short extension; externally, callers reach the same person through the full E.164 number that DID points at that extension.[6][8]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":5},{"id":"8c2a608f-21e5-43f7-aab2-c118308e03f0","sectionKey":"cost_model","sectionType":"markdown_section","heading":"How you pay for each","introMarkdown":"The cost models differ because one is a service and the other is a system you run. PSTN access is billed by a carrier, traditionally per line and per call or per minute, so you are paying for connectivity to the public network as an ongoing service.[4] A PBX changes what you are buying. Instead of a line per person, you buy or subscribe to a switch and a shared set of trunks, then pay the carrier only for those trunks and the calls that use them.[1] An on-premises PBX has historically been a capital cost, the equipment plus its maintenance, while a hosted or cloud PBX turns that into a predictable per-user monthly fee with the provider absorbing the hardware.[2] The saving that made PBXs standard is straightforward: sharing a small pool of outside lines across many phones costs far less than giving every phone its own line to the central office.[1][3]","introHtml":"The cost models differ because one is a service and the other is a system you run. PSTN access is billed by a carrier, traditionally per line and per call or per minute, so you are paying for connectivity to the public network as an ongoing service.[4] A PBX changes what you are buying. Instead of a line per person, you buy or subscribe to a switch and a shared set of trunks, then pay the carrier only for those trunks and the calls that use them.[1] An on-premises PBX has historically been a capital cost, the equipment plus its maintenance, while a hosted or cloud PBX turns that into a predictable per-user monthly fee with the provider absorbing the hardware.[2] The saving that made PBXs standard is straightforward: sharing a small pool of outside lines across many phones costs far less than giving every phone its own line to the central office.[1][3]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":6},{"id":"4ed8544d-57d5-47cc-ac6c-9f7bde3f78ba","sectionKey":"how_pbx_connects_to_pstn","sectionType":"markdown_section","heading":"How does a PBX connect to the PSTN?","introMarkdown":"A PBX reaches the PSTN through trunk lines, the shared external circuits that carry calls between the private switch and the public network.[1] When an internal caller dials an outside number, the PBX selects a free trunk, passes the call to the PSTN, and the public network handles the routing to the destination.[1][3] For decades those trunks were physical carrier circuits: analog central office lines for small systems, or a digital Primary Rate Interface delivering multiple channels over a T1 or E1 for larger ones.[6] The number of trunks sets how many outside calls can run at once, which is why capacity planning for a PBX is about concurrent calls rather than headcount.[1]\n\nMost new deployments now use a SIP trunk instead of a physical circuit. A SIP trunk carries calls as IP data to a provider that connects them to the PSTN, replacing the older analog line or PRI while doing the same job of linking the PBX to the public network.[2][10] Incoming calls travel the same path in reverse. The PSTN delivers a call to the organization's trunks, and direct inward dialing hands it to the correct extension, so an outside caller reaches an individual desk without going through a switchboard operator.[6]","introHtml":"A PBX reaches the PSTN through trunk lines, the shared external circuits that carry calls between the private switch and the public network.[1] When an internal caller dials an outside number, the PBX selects a free trunk, passes the call to the PSTN, and the public network handles the routing to the destination.[1][3] For decades those trunks were physical carrier circuits: analog central office lines for small systems, or a digital Primary Rate Interface delivering multiple channels over a T1 or E1 for larger ones.[6] The number of trunks sets how many outside calls can run at once, which is why capacity planning for a PBX is about concurrent calls rather than headcount.[1]
\nMost new deployments now use a SIP trunk instead of a physical circuit. A SIP trunk carries calls as IP data to a provider that connects them to the PSTN, replacing the older analog line or PRI while doing the same job of linking the PBX to the public network.[2][10] Incoming calls travel the same path in reverse. The PSTN delivers a call to the organization's trunks, and direct inward dialing hands it to the correct extension, so an outside caller reaches an individual desk without going through a switchboard operator.[6]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":7},{"id":"7885faed-f567-4682-8772-65886f2509d6","sectionKey":"ip_and_cloud_blur","sectionType":"markdown_section","heading":"How IP and cloud PBXs are blurring the line","introMarkdown":"The clean split between a private box and the public network has softened, because the PBX has moved into software and the cloud. An IP PBX performs the same call switching as a traditional PBX but uses digital IP signaling over the office data network rather than analog lines, so phones connect with Ethernet instead of dedicated telephone wiring.[1][2] A hosted or cloud PBX goes further, moving the switching itself into a provider's data center; the business keeps only handsets or software apps and pays a per-user subscription, while the provider runs the equipment and its connection to the PSTN.[1][2] The underlying distinction still holds. A private switching layer, wherever it physically runs, serves one organization and hands its outside calls to the shared public network, which is the same relationship a copper-era PBX had.[2][5]","introHtml":"The clean split between a private box and the public network has softened, because the PBX has moved into software and the cloud. An IP PBX performs the same call switching as a traditional PBX but uses digital IP signaling over the office data network rather than analog lines, so phones connect with Ethernet instead of dedicated telephone wiring.[1][2] A hosted or cloud PBX goes further, moving the switching itself into a provider's data center; the business keeps only handsets or software apps and pays a per-user subscription, while the provider runs the equipment and its connection to the PSTN.[1][2] The underlying distinction still holds. A private switching layer, wherever it physically runs, serves one organization and hands its outside calls to the shared public network, which is the same relationship a copper-era PBX had.[2][5]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":8},{"id":"b4206be4-e740-4b44-b294-dc05075c136c","sectionKey":"trade_offs","sectionType":"markdown_section","heading":"Trade-offs and what to watch","introMarkdown":"Choosing how to handle business calling is really a question of where you want the switching to live and how you want to reach the PSTN, and each option carries a different set of practical concerns.\n\n### On-premises PBX\n\nRunning your own PBX gives the most direct control over the dial plan and features, and internal calls stay entirely on your equipment.[1] The cost is that you own the hardware, its maintenance, and the trunks, and you carry the risk if the switch or its power fails.[2] It suits organizations that want that control and have the staff to manage it.\n\n### Hosted or cloud PBX\n\nMoving the PBX to a provider removes the hardware and shifts to a predictable per-user fee, with the provider handling upgrades and the PSTN connection.[2] The trade-off is that call quality and uptime now ride on your internet connection and the provider rather than on a dedicated telephone circuit.[4] It fits organizations that prefer an operating cost and remote-friendly access over on-site control.\n\n### The PSTN transition\n\nThe public network itself is changing underneath all of these choices. Carriers are retiring the legacy circuit-switched PSTN in favor of IP, and regulators describe the old analog network as having reached the end of its serviceable life, with landline service moving onto broadband-based voice.[4][9] The practical effect is that whatever PBX you run, its link to the outside world is increasingly a SIP or IP trunk rather than copper, so planning around IP is now the safer default.[9][10]","introHtml":"Choosing how to handle business calling is really a question of where you want the switching to live and how you want to reach the PSTN, and each option carries a different set of practical concerns.
\nRunning your own PBX gives the most direct control over the dial plan and features, and internal calls stay entirely on your equipment.[1] The cost is that you own the hardware, its maintenance, and the trunks, and you carry the risk if the switch or its power fails.[2] It suits organizations that want that control and have the staff to manage it.
\nMoving the PBX to a provider removes the hardware and shifts to a predictable per-user fee, with the provider handling upgrades and the PSTN connection.[2] The trade-off is that call quality and uptime now ride on your internet connection and the provider rather than on a dedicated telephone circuit.[4] It fits organizations that prefer an operating cost and remote-friendly access over on-site control.
\nThe public network itself is changing underneath all of these choices. Carriers are retiring the legacy circuit-switched PSTN in favor of IP, and regulators describe the old analog network as having reached the end of its serviceable life, with landline service moving onto broadband-based voice.[4][9] The practical effect is that whatever PBX you run, its link to the outside world is increasingly a SIP or IP trunk rather than copper, so planning around IP is now the safer default.[9][10]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":9},{"id":"bdc6493b-82d1-4051-b598-316b68f401e4","sectionKey":"what_they_are_not","sectionType":"markdown_section","heading":"What the PSTN and a PBX are not","introMarkdown":"A few common mix-ups blur these two, and separating them clarifies both.\n\n### A PBX is not the PSTN\n\nA PBX is a private switch for one organization, not the public network, even though it connects to it.[1][5] It routes your internal calls and shares your outside lines, while the PSTN carries calls between everyone, across carriers and borders.[5]\n\n### Neither one is VoIP by itself\n\nVoice over IP is a method of carrying calls as data, not a network or a switch.[4] A PBX can be analog or IP, and the PSTN is being rebuilt on IP, so 'VoIP' describes how calls travel rather than naming either the private switch or the public network.[2][4]\n\n### A SIP trunk is not a PBX\n\nA SIP trunk is the connection that links a PBX to the PSTN over IP, not the switching system itself.[1][10] The PBX does the routing; the SIP trunk is one way it reaches the outside world, in the same role a PRI or analog line once filled.[6][10]\n\n### Centrex is not an on-premises PBX\n\nCentrex offers PBX-like features but runs from the carrier's central exchange on the carrier's own equipment, so a business using it has no switch of its own on site.[7]","introHtml":"A few common mix-ups blur these two, and separating them clarifies both.
\nA PBX is a private switch for one organization, not the public network, even though it connects to it.[1][5] It routes your internal calls and shares your outside lines, while the PSTN carries calls between everyone, across carriers and borders.[5]
\nVoice over IP is a method of carrying calls as data, not a network or a switch.[4] A PBX can be analog or IP, and the PSTN is being rebuilt on IP, so 'VoIP' describes how calls travel rather than naming either the private switch or the public network.[2][4]
\nA SIP trunk is the connection that links a PBX to the PSTN over IP, not the switching system itself.[1][10] The PBX does the routing; the SIP trunk is one way it reaches the outside world, in the same role a PRI or analog line once filled.[6][10]
\nCentrex offers PBX-like features but runs from the carrier's central exchange on the carrier's own equipment, so a business using it has no switch of its own on site.[7]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":10},{"id":"50bc51a1-1d5d-4af3-83ce-49011c1fc94c","sectionKey":"contributor_perspective","sectionType":"markdown_section","heading":"How this answer was researched","introMarkdown":"The definitions here come from reference and industry technical sources, cross-checked against each other so that no single description carries a claim on its own. The meaning of a PBX, its trunks, and its cost logic are drawn from technical reference entries and a vendor primer, and the PSTN definition and its public, carrier-run structure come from independent reference sources.[1][2][3][4][5] The numbering distinction rests on the ITU-T standard that defines public telephone numbers and on the reference description of direct inward dialing that bridges those numbers to PBX extensions.[6][8] Claims about how IP and cloud systems connect a PBX to the public network, and about the wider retirement of the legacy PSTN, are supported by a provider explainer alongside independent and government sources rather than by any one of them alone.[7][9][10] Telephone technology and the terms around it shift over time, so the descriptions here reflect what the cited sources stated on the verification date. Practitioners who design, install, or operate these systems are welcome to suggest corrections, which are checked against primary sources before any update.","introHtml":"The definitions here come from reference and industry technical sources, cross-checked against each other so that no single description carries a claim on its own. The meaning of a PBX, its trunks, and its cost logic are drawn from technical reference entries and a vendor primer, and the PSTN definition and its public, carrier-run structure come from independent reference sources.[1][2][3][4][5] The numbering distinction rests on the ITU-T standard that defines public telephone numbers and on the reference description of direct inward dialing that bridges those numbers to PBX extensions.[6][8] Claims about how IP and cloud systems connect a PBX to the public network, and about the wider retirement of the legacy PSTN, are supported by a provider explainer alongside independent and government sources rather than by any one of them alone.[7][9][10] Telephone technology and the terms around it shift over time, so the descriptions here reflect what the cited sources stated on the verification date. Practitioners who design, install, or operate these systems are welcome to suggest corrections, which are checked against primary sources before any update.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":"This answer was written and reviewed by the AnswerStack Editorial Team, which has no commercial stake in the products, companies, or methods discussed. Every claim is cited inline and verified on the dates shown.","noteHtml":"This answer was written and reviewed by the AnswerStack Editorial Team, which has no commercial stake in the products, companies, or methods discussed. Every claim is cited inline and verified on the dates shown.
\n","sortOrder":11}],"citations":[{"title":"What is a private branch exchange (PBX)?","url":"https://www.techtarget.com/searchunifiedcommunications/definition/private-branch-exchange","excerpt":"A private branch exchange (PBX) is a telephone system within an enterprise that switches calls between users on local lines, while enabling all users to share a certain number of external phone lines.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"PBX switches calls between internal users while sharing a limited number of external phone lines; saves the cost of a line per user to the central office; trunk lines terminate at the PBX; traditional analog, IP, and hosted PBX types","domain":"techtarget.com","publisherName":"Informa TechTarget"},{"title":"Business telephone system","url":"https://en.wikipedia.org/wiki/Business_telephone_system","excerpt":"The central office lines provide connections to the public switched telephone network (PSTN) and the concentration aspect of a PBX permits the shared use of these lines between all stations in the organization.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"PBX is privately owned and operated; central office lines connect it to the PSTN and are shared across stations by concentration; each device is an extension with an extension number; VoIP and hosted PBX developments in the 1990s","domain":"en.wikipedia.org","publisherName":"Wikipedia"},{"title":"PBX Systems for Business Efficiency: A Primer","url":"https://www.mitel.com/articles/whats-the-meaning-of-pbx","excerpt":"a PBX allows a company to accommodate all its external voice communication requirements with a small number of external lines.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"A PBX lets a company meet its external voice needs with a small number of external lines; internal calls stay on the PBX; the switch routes many paths and shares limited outside lines efficiently","domain":"mitel.com","publisherName":"Mitel"},{"title":"PSTN (public switched telephone network)","url":"https://www.techtarget.com/searchnetworking/definition/PSTN","excerpt":"PSTN (public switched telephone network) is the world's collection of interconnected voice-oriented public telephone networks via traditional circuit-switched networks.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"PSTN is the world's collection of interconnected circuit-switched public telephone networks; you connect to it as a carrier service; providers are transitioning from PSTN to all-IP; VoIP carries calls without circuit switching","domain":"techtarget.com","publisherName":"Informa TechTarget"},{"title":"Public switched telephone network","url":"https://en.wikipedia.org/wiki/Public_switched_telephone_network","excerpt":"The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"PSTN is the aggregate of operator-run telephone networks; uses circuit switching; large private company networks link to the PSTN only through limited gateways such as a large PBX","domain":"en.wikipedia.org","publisherName":"Wikipedia"},{"title":"Direct inward dialing","url":"https://en.wikipedia.org/wiki/Direct_inward_dial","excerpt":"the telephone company provides one or more trunk lines to the customer for connection to the customer's PBX, and allocates a range of telephone numbers to the customer.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"DID is a carrier service for PBX operators; the telephone company provides trunk lines and allocates a range of numbers; the dialed number is signaled to the PBX so a call routes to an extension without an operator; fewer circuits than DID numbers","domain":"en.wikipedia.org","publisherName":"Wikipedia"},{"title":"Centrex","url":"https://en.wikipedia.org/wiki/Centrex","excerpt":"It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"Centrex provides PBX-like functions but with equipment owned by and located at the telephone company, delivered from the central exchange rather than on the customer premises","domain":"en.wikipedia.org","publisherName":"Wikipedia"},{"title":"ITU-T E.164: The international public telecommunication numbering plan","url":"https://www.itu.int/rec/T-REC-E.164/","excerpt":"The international public telecommunication numbering plan. Status: In force.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"E.164 is the international public telecommunication numbering plan governing public telephone numbers worldwide; maximum 15 digits with country codes; current version in force","domain":"itu.int","publisherName":"International Telecommunication Union (ITU-T)"},{"title":"UK transition from analogue to digital landlines","url":"https://www.gov.uk/guidance/uk-transition-from-analogue-to-digital-landlines","excerpt":"Analogue networks have been in operation for decades and have reached the end of their serviceable life.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"The analog PSTN has reached the end of its serviceable life; the industry is upgrading landline services to digital technology using an internet connection such as VoIP and All-IP telephony","domain":"gov.uk","publisherName":"GOV.UK"},{"title":"SIP Trunking vs. Hosted PBX: 5 Key Differences","url":"https://www.atlantech.net/blog/sip-trunking-vs.-hosted-pbx-whats-the-difference","excerpt":"SIP Trunking uses IP at the application layer to connect a phone call to the Public Switched Telephone Network (PSTN), replacing a traditional 'phone trunk' such as a Primary Rate Interface (PRI) or analog line.","quoteText":null,"sourceRole":"SUPPORTING","verifiedAt":"2026-07-18T00:00:00","supportsText":"A SIP trunk connects a PBX to the PSTN over IP, replacing a traditional phone trunk such as a PRI or analog line; corroborated by [2] on IP and hosted PBX connectivity","domain":"atlantech.net","publisherName":"Atlantech Online"}],"revisions":[],"relatedAnswers":[{"id":"3cc4d7a6-6aed-4446-8dd8-13e022c3e1ba","slug":"is-pstn-secure-and-can-pstn-calls-be-encrypted","question":"Is PSTN secure, and can PSTN calls be encrypted?","publishedAt":"2026-07-20T16:01:27.137","confidenceScore":92,"confidenceLabel":"High","industry":{"id":"ff619d7c-d7d7-485e-a05a-53fba07f33ed","slug":"telecommunications","label":"Telecommunications","description":"Business voice, fiber, UCaaS, and network services"},"topic":{"slug":"pstn","label":"PSTN","description":"The public switched telephone network: how it works, POTS retirement, and the move to VoIP.","schemaKind":null},"contributor":{"id":"ec39deab-44fe-48d8-9029-fefe993ab85a","slug":"answer-stack","displayName":"AnswerStack","websiteUrl":null},"snippet":"Standard PSTN calls are unencrypted, so they are exposed to line taps, interception inside the exchange (which carriers are legally required to support), and SS7 signaling attacks that can redirect calls and track users. The network supplies no encryption of its own. Confidentiality is possible only by adding matched crypto devices at both ends, such as STU-III or STE secure phones, or by carrying the call over encrypted IP using SRTP and TLS. Encryption always lives at the endpoints, never in the PSTN itself.","url":"/q/is-pstn-secure-and-can-pstn-calls-be-encrypted"},{"id":"895bda56-c48a-429f-8f13-53dc29c09b67","slug":"which-legacy-devices-are-still-at-risk-when-pstn-pots-lines-are-retired-or","question":"Which legacy devices are still at risk when PSTN/POTS lines are retired or moved to VoIP?","publishedAt":"2026-07-20T16:01:24.898","confidenceScore":90,"confidenceLabel":"High","industry":{"id":"ff619d7c-d7d7-485e-a05a-53fba07f33ed","slug":"telecommunications","label":"Telecommunications","description":"Business voice, fiber, UCaaS, and network services"},"topic":{"slug":"pstn","label":"PSTN","description":"The public switched telephone network: how it works, POTS retirement, and the move to VoIP.","schemaKind":null},"contributor":{"id":"ec39deab-44fe-48d8-9029-fefe993ab85a","slug":"answer-stack","displayName":"AnswerStack","websiteUrl":null},"snippet":"When copper POTS lines move to VoIP, the equipment that fails is the kind that dials out as data or reaches for help automatically: fire alarm panel communicators, elevator emergency phones, burglar alarm dialers, medical alert pendants, fax machines, and card-payment terminals. They break because packet networks compress and drop the exact tones these machines depend on, and because a VoIP line loses power in an outage that a copper loop survived. Each one needs a cellular, IP, or analog-emulating replacement before its line is cut.","url":"/q/which-legacy-devices-are-still-at-risk-when-pstn-pots-lines-are-retired-or"},{"id":"28c898f5-b2d9-4a9d-88be-bd46bb9848e9","slug":"what-is-a-pstn-gateway-and-what-do-fxs-and-fxo-ports-do","question":"What is a PSTN gateway, and what do FXS and FXO ports do?","publishedAt":"2026-07-20T16:01:22.747","confidenceScore":93,"confidenceLabel":"High","industry":{"id":"ff619d7c-d7d7-485e-a05a-53fba07f33ed","slug":"telecommunications","label":"Telecommunications","description":"Business voice, fiber, UCaaS, and network services"},"topic":{"slug":"pstn","label":"PSTN","description":"The public switched telephone network: how it works, POTS retirement, and the move to VoIP.","schemaKind":null},"contributor":{"id":"ec39deab-44fe-48d8-9029-fefe993ab85a","slug":"answer-stack","displayName":"AnswerStack","websiteUrl":null},"snippet":"A PSTN gateway bridges the traditional phone network and a VoIP system by converting analog or TDM voice into IP packets and back. Its analog ports come in two types: an FXS port powers and rings an end device like a phone or fax, and an FXO port connects to a live phone line from the PSTN or a PBX. FXS and FXO always pair, one feeding the line the other receives.","url":"/q/what-is-a-pstn-gateway-and-what-do-fxs-and-fxo-ports-do"},{"id":"5b57d468-46b4-435f-b89f-606f48f3ce0b","slug":"what-is-the-difference-between-pstn-and-pots","question":"What is the difference between PSTN and POTS?","publishedAt":"2026-07-18T15:32:02.968","confidenceScore":92,"confidenceLabel":"High","industry":{"id":"ff619d7c-d7d7-485e-a05a-53fba07f33ed","slug":"telecommunications","label":"Telecommunications","description":"Business voice, fiber, UCaaS, and network services"},"topic":{"slug":"pstn","label":"PSTN","description":"The public switched telephone network: how it works, POTS retirement, and the move to VoIP.","schemaKind":null},"contributor":{"id":"ec39deab-44fe-48d8-9029-fefe993ab85a","slug":"answer-stack","displayName":"AnswerStack","websiteUrl":null},"snippet":"POTS and the PSTN describe two layers of the same telephone system. Plain old telephone service is the basic analog voice product on a single copper line, while the PSTN is the whole switched network of exchanges and trunks that carries the call. POTS runs on the PSTN rather than beside it, which is why the two terms get used interchangeably. The core of the PSTN is digital, while POTS stays analog at the edge, and both are now being retired as voice moves to internet protocol.","url":"/q/what-is-the-difference-between-pstn-and-pots"}],"contributorStats":{"verifiedAnswers":224,"openDisputes":0},"schemaJson":{"@context":"https://schema.org","@type":"Question","name":"What is the difference between PSTN and PBX?","text":"What is the difference between PSTN and PBX?","url":"https://www.answerstack.io/q/what-is-the-difference-between-pstn-and-pbx","answerCount":1,"datePublished":"2026-07-18T15:32:04.773","author":{"@type":"Person","name":"AnswerStack Editorial Team","worksFor":{"@type":"Organization","name":"AnswerStack"},"url":"https://www.answerstack.io/contributors/answer-stack"},"about":[{"@type":"Thing","name":"PSTN"},{"@type":"Thing","name":"Telecommunications"}],"acceptedAnswer":{"@type":"Answer","text":"The PSTN and a PBX operate at different scales. The public switched telephone network (PSTN) is the shared, carrier-operated system of interconnected telephone networks that can reach almost any phone in the world,[4][5] while a private branch exchange (PBX) is a single organization's own telephone switch that routes calls between its internal users and shares a small number of outside lines among them.[1] The PSTN belongs to national, regional, and local operators and works as public infrastructure,[5] whereas a PBX is privately owned and usually sits on a company's premises or, increasingly, in a provider's cloud.[1][2] The two are not competitors but layers that connect: a PBX links to the PSTN through trunk lines so its extensions can call the outside world, which is why an office needs far fewer external lines than it has phones.[1][3] Modern IP PBX and hosted PBX systems perform the same switching in software and often reach the PSTN over a SIP trunk instead of copper.[2][10]","url":"https://www.answerstack.io/q/what-is-the-difference-between-pstn-and-pbx","upvoteCount":0,"datePublished":"2026-07-18T15:32:04.773","dateModified":"2026-07-18T00:00:00","author":{"@type":"Person","name":"AnswerStack Editorial Team","worksFor":{"@type":"Organization","name":"AnswerStack"},"url":"https://www.answerstack.io/contributors/answer-stack"},"citation":[{"@type":"CreativeWork","name":"What is a private branch exchange (PBX)?","url":"https://www.techtarget.com/searchunifiedcommunications/definition/private-branch-exchange"},{"@type":"CreativeWork","name":"Business telephone system","url":"https://en.wikipedia.org/wiki/Business_telephone_system"},{"@type":"CreativeWork","name":"PBX Systems for Business Efficiency: A Primer","url":"https://www.mitel.com/articles/whats-the-meaning-of-pbx"},{"@type":"CreativeWork","name":"PSTN (public switched telephone network)","url":"https://www.techtarget.com/searchnetworking/definition/PSTN"},{"@type":"CreativeWork","name":"Public switched telephone network","url":"https://en.wikipedia.org/wiki/Public_switched_telephone_network"},{"@type":"CreativeWork","name":"Direct inward dialing","url":"https://en.wikipedia.org/wiki/Direct_inward_dial"},{"@type":"CreativeWork","name":"Centrex","url":"https://en.wikipedia.org/wiki/Centrex"},{"@type":"CreativeWork","name":"ITU-T E.164: The international public telecommunication numbering plan","url":"https://www.itu.int/rec/T-REC-E.164/"},{"@type":"CreativeWork","name":"UK transition from analogue to digital landlines","url":"https://www.gov.uk/guidance/uk-transition-from-analogue-to-digital-landlines"},{"@type":"CreativeWork","name":"SIP Trunking vs. Hosted PBX: 5 Key Differences","url":"https://www.atlantech.net/blog/sip-trunking-vs.-hosted-pbx-whats-the-difference"}]}}}