The public switched telephone network, or PSTN, is the aggregate of the world's interconnected, operator-run telephone networks, and it has carried ordinary landline calls for well over a century.[1] It works by circuit switching, meaning the network reserves a dedicated end-to-end path for each call and holds it open for the whole conversation, which is the main way it differs from the packet networks that carry internet traffic.[2] A call travels from your phone over a local loop to a local exchange, then across shared trunks and higher-level switches to the exchange serving the person you are calling, with a separate control network called SS7 setting up and releasing the connection.[1][4] Since the 1960s the core of the network has been digital, encoding each voice channel as a 64 kbit/s stream and multiplexing many of them onto shared circuits.[3][7] Carriers are now retiring the PSTN in favor of internet protocol networks, and in the United Kingdom the switch off is scheduled for 31 January 2027.[8][9]
\n","summary":"The PSTN is the worldwide system of interconnected, operator-run telephone networks that has delivered landline calls for more than a century using circuit switching, which reserves a dedicated path for each call. A call runs from a copper local loop to a local exchange, across trunks and tandem switches, and back down to the far end, with SS7 handling signaling and ITU-T E.164 governing the numbers. Carriers are replacing it with internet protocol networks, and the UK plans to switch its PSTN off by 31 January 2027.","publishedAt":"2026-07-18T15:17:01.345","verifiedAt":"2026-07-18T00:00:00","editorialStatus":"APPROVED","lastReviewedAt":"2026-07-18T00:00:00","nextReviewDueAt":"2026-10-18T00:00:00","templateVersion":"v2","aliases":["What does PSTN stand for?","What is the public switched telephone network?","PSTN meaning","How does the PSTN work?","What is a landline network?","What is the traditional telephone network?","Circuit-switched telephone network explained","PSTN vs VoIP","What is POTS?","Is the PSTN being switched off?","What is the difference between PSTN and VoIP?","How are phone calls routed on 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":"9591aa2d-e5fc-4b97-813c-278cf3f4d339","sectionKey":"how_pstn_works","sectionType":"markdown_section","heading":"How does the PSTN work?","introMarkdown":"The PSTN carries a phone call by building a temporary, dedicated circuit between two telephones and keeping that circuit open until someone hangs up.[2] When you lift a handset, your phone forms a connection over a pair of copper wires, the local loop, to the nearest telephone exchange, also called a central office in North American usage.[1] That exchange reads the number you dial and either connects you to another line it serves directly, or hands the call up to higher-level switches that route it across the country or the world.[1] The path your voice takes is reserved for you alone for the length of the call, which is the defining trait of circuit switching and the main way the PSTN differs from the packet networks behind the internet.[2]\n\nThe network is arranged as a hierarchy so that it does not need a direct wire between every pair of exchanges.[1] Local exchanges connect to each other and to larger tandem and toll switches through shared links called trunks, and a long distance call climbs that hierarchy only as far as it needs to before it comes back down to the destination exchange.[1] This structure keeps local traffic local and reserves the costly long-haul circuits for calls that actually cross regions.\n\n### A worldwide system, not one company\n\nNo single organization owns the PSTN. It is the combined result of many national, regional, and local carriers interconnecting their networks under shared technical standards, most of them set by the International Telecommunication Union, so that a call placed in one country can reach a phone in another.[1][5] Those standards cover how numbers are formatted, how calls are signaled between networks, and how voice is encoded once it becomes digital, which is what lets equipment from different vendors and different countries work together as a single global network.","introHtml":"The PSTN carries a phone call by building a temporary, dedicated circuit between two telephones and keeping that circuit open until someone hangs up.[2] When you lift a handset, your phone forms a connection over a pair of copper wires, the local loop, to the nearest telephone exchange, also called a central office in North American usage.[1] That exchange reads the number you dial and either connects you to another line it serves directly, or hands the call up to higher-level switches that route it across the country or the world.[1] The path your voice takes is reserved for you alone for the length of the call, which is the defining trait of circuit switching and the main way the PSTN differs from the packet networks behind the internet.[2]
\nThe network is arranged as a hierarchy so that it does not need a direct wire between every pair of exchanges.[1] Local exchanges connect to each other and to larger tandem and toll switches through shared links called trunks, and a long distance call climbs that hierarchy only as far as it needs to before it comes back down to the destination exchange.[1] This structure keeps local traffic local and reserves the costly long-haul circuits for calls that actually cross regions.
\nNo single organization owns the PSTN. It is the combined result of many national, regional, and local carriers interconnecting their networks under shared technical standards, most of them set by the International Telecommunication Union, so that a call placed in one country can reach a phone in another.[1][5] Those standards cover how numbers are formatted, how calls are signaled between networks, and how voice is encoded once it becomes digital, which is what lets equipment from different vendors and different countries work together as a single global network.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":0},{"id":"c4f1ca12-0949-4b1c-932b-477751573ce2","sectionKey":"pstn_digital","sectionType":"markdown_section","heading":"How did the PSTN become a digital network?","introMarkdown":"Although the local line to a home is often still analog, the core of the PSTN has been digital since the 1960s, when carriers began converting voice into a stream of numbers rather than a continuous electrical wave.[1] The conversion uses pulse code modulation, sampling the sound of a voice 8,000 times a second and coding each sample as an 8 bit value, which produces a single voice channel of 64 kbit/s.[3][7] The ITU-T standard that describes this encoding, G.711, still defines the baseline sound quality most people associate with a normal phone call.[3]\n\nThat 64 kbit/s channel, known as a DS0, is the basic unit the network multiplexes onto shared circuits.[7] In North America a T1 line combines 24 of these channels into a 1.544 Mbit/s signal, while much of the rest of the world uses an E1 that carries 32 timeslots at 2.048 Mbit/s, of which 30 carry voice.[7] Higher up, carriers aggregate thousands of calls onto fiber using synchronous transport systems such as SONET and SDH.[1] The switching stayed circuit based through all of this, so even a fully digital call still holds a fixed 64 kbit/s slot from end to end rather than sharing capacity the way internet traffic does.","introHtml":"Although the local line to a home is often still analog, the core of the PSTN has been digital since the 1960s, when carriers began converting voice into a stream of numbers rather than a continuous electrical wave.[1] The conversion uses pulse code modulation, sampling the sound of a voice 8,000 times a second and coding each sample as an 8 bit value, which produces a single voice channel of 64 kbit/s.[3][7] The ITU-T standard that describes this encoding, G.711, still defines the baseline sound quality most people associate with a normal phone call.[3]
\nThat 64 kbit/s channel, known as a DS0, is the basic unit the network multiplexes onto shared circuits.[7] In North America a T1 line combines 24 of these channels into a 1.544 Mbit/s signal, while much of the rest of the world uses an E1 that carries 32 timeslots at 2.048 Mbit/s, of which 30 carry voice.[7] Higher up, carriers aggregate thousands of calls onto fiber using synchronous transport systems such as SONET and SDH.[1] The switching stayed circuit based through all of this, so even a fully digital call still holds a fixed 64 kbit/s slot from end to end rather than sharing capacity the way internet traffic does.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":1},{"id":"e1267d1b-ad03-4e62-8336-ca17399787bd","sectionKey":"building_blocks_table","sectionType":"table_section","heading":"The main building blocks of the PSTN","introMarkdown":"Each part of the PSTN plays a distinct role in getting a call from one phone to another. The table lists the main building blocks, and the sections after it explain what each one does and why it matters.","introHtml":"Each part of the PSTN plays a distinct role in getting a call from one phone to another. The table lists the main building blocks, and the sections after it explain what each one does and why it matters.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{"rows":[{"cells":["Local loop","The access line, usually a copper pair, from a phone to its local exchange","Connects each subscriber to the network [1]"]},{"cells":["Local exchange (central office)","The switch that serves the lines in an area and connects their calls","Where your line terminates and calls are set up [1]"]},{"cells":["Trunks and switching hierarchy","Shared links and higher-level switches between exchanges and regions","Carry calls between sites without a wire per pair [1]"]},{"cells":["SS7 signaling","A separate control network standardized in the ITU-T Q.700 series","Sets up, routes, and releases calls out of band [4]"]},{"cells":["Numbering plan","ITU-T E.164 and national schemes such as the NANP","Give every line a unique, routable number [5]"]}],"columns":["Building block","What it is","Role in a call"]},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":2},{"id":"46f53ac4-3e43-4a19-a4de-766bec18ce62","sectionKey":"local_loop","sectionType":"markdown_section","heading":"The local loop","introMarkdown":"The local loop is the physical line that connects a single telephone to its local exchange, and for most of the PSTN's history it has been a pair of copper wires running from the premises to the nearest central office.[1] This segment is often called the last mile, and it is the part of the network a subscriber actually holds a connection to. One practical feature of the traditional copper loop is that it carries its own electrical power from the exchange, which is why an old wired phone usually kept working during a local power cut.[9] As carriers replace copper with fiber and IP, that self-powered property goes away, so the replacement services need their own backup power to keep emergency calls working, a point regulators have made central to the switch off.[9]","introHtml":"The local loop is the physical line that connects a single telephone to its local exchange, and for most of the PSTN's history it has been a pair of copper wires running from the premises to the nearest central office.[1] This segment is often called the last mile, and it is the part of the network a subscriber actually holds a connection to. One practical feature of the traditional copper loop is that it carries its own electrical power from the exchange, which is why an old wired phone usually kept working during a local power cut.[9] As carriers replace copper with fiber and IP, that self-powered property goes away, so the replacement services need their own backup power to keep emergency calls working, a point regulators have made central to the switch off.[9]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":3},{"id":"454be3ec-edec-45a2-aae5-816fb2336a0b","sectionKey":"local_exchange","sectionType":"markdown_section","heading":"The local exchange, or central office","introMarkdown":"A local exchange, called a central office in North American usage, is the building and switching equipment that terminates the local loops in an area and connects calls between them.[1] When you dial a number, this switch reads the digits and either completes the call to another line it serves or passes it toward the correct destination.[2] In the traditional numbering scheme the exchange is identified by the prefix in the middle of a phone number, which is why lines in the same neighborhood often share those digits.[6] Engineers historically called this a Class 5 switch, the lowest tier of the hierarchy and the one closest to the subscriber.[1] Because both the calling and called lines depend on their local exchanges, a call can only be completed when the exchanges at both ends are in service.","introHtml":"A local exchange, called a central office in North American usage, is the building and switching equipment that terminates the local loops in an area and connects calls between them.[1] When you dial a number, this switch reads the digits and either completes the call to another line it serves or passes it toward the correct destination.[2] In the traditional numbering scheme the exchange is identified by the prefix in the middle of a phone number, which is why lines in the same neighborhood often share those digits.[6] Engineers historically called this a Class 5 switch, the lowest tier of the hierarchy and the one closest to the subscriber.[1] Because both the calling and called lines depend on their local exchanges, a call can only be completed when the exchanges at both ends are in service.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":4},{"id":"2283018a-2402-42f8-9a54-1a6760142e85","sectionKey":"trunks_hierarchy","sectionType":"markdown_section","heading":"Trunks and the switching hierarchy","introMarkdown":"Trunks are the shared, high-capacity links that connect exchanges to each other, and they let the PSTN avoid running a separate wire between every possible pair of phones.[1] Each local exchange connects to a limited number of trunks, so a call that cannot be completed locally travels over them to higher-level switches.[1] Those higher tiers, historically called tandem switches for regional traffic and toll or Class 4 switches for long distance, route a call across the hierarchy only as far as it needs to go before it comes back down to the destination exchange.[1] Because many calls share the same trunks, carriers size them for expected simultaneous use rather than for the total number of phones, the same capacity logic that later carried over to counting concurrent call paths on IP trunks.[2] This tiered design lets one network handle both a call between neighbors and a call between continents, with each using only as much of the hierarchy as its distance requires.","introHtml":"Trunks are the shared, high-capacity links that connect exchanges to each other, and they let the PSTN avoid running a separate wire between every possible pair of phones.[1] Each local exchange connects to a limited number of trunks, so a call that cannot be completed locally travels over them to higher-level switches.[1] Those higher tiers, historically called tandem switches for regional traffic and toll or Class 4 switches for long distance, route a call across the hierarchy only as far as it needs to go before it comes back down to the destination exchange.[1] Because many calls share the same trunks, carriers size them for expected simultaneous use rather than for the total number of phones, the same capacity logic that later carried over to counting concurrent call paths on IP trunks.[2] This tiered design lets one network handle both a call between neighbors and a call between continents, with each using only as much of the hierarchy as its distance requires.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":5},{"id":"30810d47-282f-4ef8-8f2b-2653ff9b9b55","sectionKey":"ss7_signaling","sectionType":"markdown_section","heading":"SS7 signaling and how calls are set up","introMarkdown":"Signaling System No. 7, or SS7, is the separate control network the PSTN uses to set up, manage, and end calls, and it runs on its own channels rather than on the line that carries your voice.[4] Standardized by the ITU-T in its Q.700 series, SS7 does the behind-the-scenes work of a call: it passes the dialed number between switches, selects a route, rings the far end, and releases the circuit once the call is over.[4] Keeping this signaling out of band, on a dedicated data network, let calls set up faster and freed the voice path to carry only the conversation.[1] The same signaling network also made possible services callers now expect, such as caller ID and number portability, because switches can query shared databases before a call connects.[1]","introHtml":"Signaling System No. 7, or SS7, is the separate control network the PSTN uses to set up, manage, and end calls, and it runs on its own channels rather than on the line that carries your voice.[4] Standardized by the ITU-T in its Q.700 series, SS7 does the behind-the-scenes work of a call: it passes the dialed number between switches, selects a route, rings the far end, and releases the circuit once the call is over.[4] Keeping this signaling out of band, on a dedicated data network, let calls set up faster and freed the voice path to carry only the conversation.[1] The same signaling network also made possible services callers now expect, such as caller ID and number portability, because switches can query shared databases before a call connects.[1]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":6},{"id":"be7c822f-4c55-4d87-9024-3a0a69e5ccda","sectionKey":"numbering_plan","sectionType":"markdown_section","heading":"How are phones on the PSTN numbered?","introMarkdown":"Every telephone on the PSTN is reachable through a structured number, and the master framework for those numbers is ITU-T Recommendation E.164, the international public telecommunication numbering plan.[5] E.164 limits an international number to a maximum of 15 digits and begins it with a country code of one to three digits that identifies the destination country or global service.[5] Within that framework each region runs its own national plan. North America uses the North American Numbering Plan, a shared scheme covering the United States, Canada, and 18 other countries and territories under a single country code of 1, with local numbers written as a three digit area code, a three digit central office prefix, and a four digit line number.[6] A neutral administrator assigns these numbering resources so that no two lines collide and so that any switch can route a call using only the digits dialed.[6]","introHtml":"Every telephone on the PSTN is reachable through a structured number, and the master framework for those numbers is ITU-T Recommendation E.164, the international public telecommunication numbering plan.[5] E.164 limits an international number to a maximum of 15 digits and begins it with a country code of one to three digits that identifies the destination country or global service.[5] Within that framework each region runs its own national plan. North America uses the North American Numbering Plan, a shared scheme covering the United States, Canada, and 18 other countries and territories under a single country code of 1, with local numbers written as a three digit area code, a three digit central office prefix, and a four digit line number.[6] A neutral administrator assigns these numbering resources so that no two lines collide and so that any switch can route a call using only the digits dialed.[6]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":7},{"id":"8ae54da4-e5cf-43c9-b0dd-ffcf23b1bc51","sectionKey":"pstn_switch_off","sectionType":"markdown_section","heading":"Is the PSTN being switched off?","introMarkdown":"Yes. Telephone companies around the world are retiring the circuit-switched PSTN and moving voice onto internet protocol networks, because the older equipment is increasingly expensive to maintain and staff.[2][9] The clearest deadline is in the United Kingdom, where BT Group plans to retire its PSTN by 31 January 2027, after which landline calls run over broadband as Voice over IP.[8] Openreach, which operates the physical network most UK providers rely on, has been withdrawing the products that depend on the old network and has placed most exchanges under a stop sell that blocks new copper-based orders.[8] The UK government describes the analog network as having reached the end of its serviceable life and frames the migration as an industry-led program expected to be substantially complete by that date.[9]\n\n### What changes for a phone line\n\nFor most people the phone number and handset stay the same, and the call simply travels over a broadband connection instead of a dedicated copper circuit.[9] The difference regulators stress is power. Because a digital line does not draw power from the exchange the way copper did, providers in the UK are required to make sure customers can still reach emergency services during a power cut, which usually means offering a battery backup unit that supports at least an hour of calling.[9] In the United States the same shift is underway under the general heading of the technology transition, with carriers replacing copper and legacy time-division equipment with IP and fiber over a longer, less fixed timeline.[2]","introHtml":"Yes. Telephone companies around the world are retiring the circuit-switched PSTN and moving voice onto internet protocol networks, because the older equipment is increasingly expensive to maintain and staff.[2][9] The clearest deadline is in the United Kingdom, where BT Group plans to retire its PSTN by 31 January 2027, after which landline calls run over broadband as Voice over IP.[8] Openreach, which operates the physical network most UK providers rely on, has been withdrawing the products that depend on the old network and has placed most exchanges under a stop sell that blocks new copper-based orders.[8] The UK government describes the analog network as having reached the end of its serviceable life and frames the migration as an industry-led program expected to be substantially complete by that date.[9]
\nFor most people the phone number and handset stay the same, and the call simply travels over a broadband connection instead of a dedicated copper circuit.[9] The difference regulators stress is power. Because a digital line does not draw power from the exchange the way copper did, providers in the UK are required to make sure customers can still reach emergency services during a power cut, which usually means offering a battery backup unit that supports at least an hour of calling.[9] In the United States the same shift is underway under the general heading of the technology transition, with carriers replacing copper and legacy time-division equipment with IP and fiber over a longer, less fixed timeline.[2]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":8},{"id":"aabb036a-6acd-473d-b8b8-7ab8f56f44e8","sectionKey":"what_pstn_is_not","sectionType":"markdown_section","heading":"What the PSTN is not","introMarkdown":"The PSTN is the traditional telephone network, not the internet and not any single company's phone service, and a few common mix-ups are worth separating.\n\n### It is not VoIP\n\nVoice over IP carries calls as data packets over the internet or a private IP network, while the PSTN reserves a dedicated circuit for each call.[2] The two connect through gateways, so a VoIP call can reach a PSTN number and the reverse, but they are different technologies, and the industry is now moving traffic from the first onto the second.[2]\n\n### It is not the mobile network\n\nCellular networks are their own radio-access systems, though they interconnect with the PSTN and share its numbering.[5] A mobile call may travel partly over the traditional network when it reaches a landline, but the wireless portion uses different technology and, on modern phones, often carries voice as IP data inside the carrier's network.\n\n### It is not just copper wires\n\nPeople picture the PSTN as copper phone lines, and the local loop often is copper, but the core has long been digital fiber and microwave carrying millions of calls as 64 kbit/s channels.[1][7] Retiring the PSTN is really about replacing the circuit-switched method of connecting calls, not simply pulling out copper.\n\n### It is not the same as POTS\n\nPlain old telephone service, or POTS, refers to the basic analog voice service delivered to a subscriber over the local loop, which is one service the PSTN provides rather than a synonym for the whole network.[1]","introHtml":"The PSTN is the traditional telephone network, not the internet and not any single company's phone service, and a few common mix-ups are worth separating.
\nVoice over IP carries calls as data packets over the internet or a private IP network, while the PSTN reserves a dedicated circuit for each call.[2] The two connect through gateways, so a VoIP call can reach a PSTN number and the reverse, but they are different technologies, and the industry is now moving traffic from the first onto the second.[2]
\nCellular networks are their own radio-access systems, though they interconnect with the PSTN and share its numbering.[5] A mobile call may travel partly over the traditional network when it reaches a landline, but the wireless portion uses different technology and, on modern phones, often carries voice as IP data inside the carrier's network.
\nPeople picture the PSTN as copper phone lines, and the local loop often is copper, but the core has long been digital fiber and microwave carrying millions of calls as 64 kbit/s channels.[1][7] Retiring the PSTN is really about replacing the circuit-switched method of connecting calls, not simply pulling out copper.
\nPlain old telephone service, or POTS, refers to the basic analog voice service delivered to a subscriber over the local loop, which is one service the PSTN provides rather than a synonym for the whole network.[1]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":9},{"id":"22a4cc62-b31a-4e29-8aa6-e5be057a1977","sectionKey":"trade_offs","sectionType":"markdown_section","heading":"Trade-offs and limits worth knowing","introMarkdown":"The PSTN's strengths and weaknesses both come from its circuit-switched design, and naming them explains why it lasted so long and why it is now being replaced.\n\n### Reliability and voice quality\n\nReserving a dedicated path for each call gives the PSTN very consistent voice quality and call setup, because the capacity for a conversation is guaranteed the moment the call connects rather than shared with other traffic.[2] The traditional copper line also carried its own power from the exchange, so it typically kept working during a local blackout, which is the property the digital replacements have to recreate with backup batteries.[9]\n\n### Efficiency and flexibility\n\nHolding a full circuit open for every call, including the silences, uses network capacity less efficiently than packet switching, which sends data only when there is something to send.[2] The PSTN was also built for voice, so adding new features usually meant changing the switches themselves, a slow process compared with software-based IP systems.[1]\n\n### Cost of maintenance\n\nThe equipment at the heart of the network is decades old, and the pool of engineers who maintain it is shrinking, which is a large part of why carriers have decided that replacing it is cheaper than keeping it running.[9] For an organization still on the PSTN, the practical implication is that the timing of a migration is increasingly set by the carrier's retirement schedule rather than left entirely to the customer.[8]","introHtml":"The PSTN's strengths and weaknesses both come from its circuit-switched design, and naming them explains why it lasted so long and why it is now being replaced.
\nReserving a dedicated path for each call gives the PSTN very consistent voice quality and call setup, because the capacity for a conversation is guaranteed the moment the call connects rather than shared with other traffic.[2] The traditional copper line also carried its own power from the exchange, so it typically kept working during a local blackout, which is the property the digital replacements have to recreate with backup batteries.[9]
\nHolding a full circuit open for every call, including the silences, uses network capacity less efficiently than packet switching, which sends data only when there is something to send.[2] The PSTN was also built for voice, so adding new features usually meant changing the switches themselves, a slow process compared with software-based IP systems.[1]
\nThe equipment at the heart of the network is decades old, and the pool of engineers who maintain it is shrinking, which is a large part of why carriers have decided that replacing it is cheaper than keeping it running.[9] For an organization still on the PSTN, the practical implication is that the timing of a migration is increasingly set by the carrier's retirement schedule rather than left entirely to the customer.[8]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":10},{"id":"b5067a69-6b0a-4567-acc5-4fe09d5ad833","sectionKey":"contributor_perspective","sectionType":"markdown_section","heading":"How this answer was researched","introMarkdown":"This entry draws its technical claims from the standards that define the network and its regulatory and retirement claims from the bodies managing the transition. The numbering rules come from ITU-T Recommendation E.164 and the North American Numbering Plan administrator, the signaling description from the ITU-T Q.700 series, and the digital voice figures from ITU-T G.711 and an independent description of the digital hierarchy.[3][4][5][6][7] The structural overview, circuit switching, and the distinction from VoIP are drawn from reference and industry sources and cross-checked against each other.[1][2] The switch off timeline is taken from the network operator and the UK government guidance that set and describe the 31 January 2027 date, rather than from secondary summaries.[8][9] Telephone networks and their retirement schedules change, so the dates here reflect what the cited sources stated on the verification date. Engineers, regulators, and carriers who work on these systems are welcome to suggest corrections, which are checked against primary sources before any update.","introHtml":"This entry draws its technical claims from the standards that define the network and its regulatory and retirement claims from the bodies managing the transition. The numbering rules come from ITU-T Recommendation E.164 and the North American Numbering Plan administrator, the signaling description from the ITU-T Q.700 series, and the digital voice figures from ITU-T G.711 and an independent description of the digital hierarchy.[3][4][5][6][7] The structural overview, circuit switching, and the distinction from VoIP are drawn from reference and industry sources and cross-checked against each other.[1][2] The switch off timeline is taken from the network operator and the UK government guidance that set and describe the 31 January 2027 date, rather than from secondary summaries.[8][9] Telephone networks and their retirement schedules change, so the dates here reflect what the cited sources stated on the verification date. Engineers, regulators, and carriers who work on 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":"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":"Definition of the PSTN as the aggregate of operator-run telephone networks; structure of local loops, exchanges, trunks, and the switching hierarchy; digital core with PCM, DS0, SONET/SDH; SS7 controlling calls between exchanges; ITU standards","domain":"en.wikipedia.org","publisherName":"Wikipedia"},{"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 as interconnected circuit-switched networks; dedicated path per call for the call duration; hierarchical local, trunk, and main exchanges; providers transitioning to all-IP; comparison with VoIP","domain":"techtarget.com","publisherName":"Informa TechTarget"},{"title":"ITU-T G.711: Pulse code modulation (PCM) of voice frequencies","url":"https://www.itu.int/rec/T-REC-G.711/","excerpt":"Pulse code modulation (PCM) of voice frequencies. Status: In force.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"G.711 defines pulse code modulation encoding of voice frequencies, the baseline digital voice channel of the telephone network; in force","domain":"itu.int","publisherName":"International Telecommunication Union (ITU-T)"},{"title":"ITU-T Q.700: Introduction to CCITT Signalling System No. 7","url":"https://www.itu.int/rec/T-REC-Q.700/","excerpt":"Introduction to CCITT Signalling System No. 7","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"Signalling System No. 7 (SS7) as the out-of-band signaling framework that sets up, routes, and releases PSTN calls; standardized in the Q.700 series; in force","domain":"itu.int","publisherName":"International Telecommunication Union (ITU-T)"},{"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","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"E.164 is the international public telecommunication numbering plan; maximum 15 digits; country code of one to three digits; governs numbering across the worldwide PSTN; in force, replaces E.163","domain":"itu.int","publisherName":"International Telecommunication Union (ITU-T)"},{"title":"About NANPA","url":"https://www.nanpa.com/about","excerpt":"NANPA holds overall responsibility for the neutral administration of NANP numbering resources, subject to directives from regulatory authorities in the countries that share the NANP.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"North American Numbering Plan structure: country code 1, NPA-NXX-XXXX ten-digit format, 20 participating countries and territories, neutral administration of numbering resources","domain":"nanpa.com","publisherName":"North American Numbering Plan Administrator (NANPA)"},{"title":"The Plesiochronous Digital Hierarchy","url":"https://www.technologyuk.net/telecommunications/communication-technologies/plesiochronous-digital-hierarchy.shtml","excerpt":"The digitisation of an analogue voice channel into a 64 kbps digital channel (designated as digital signalling level zero or simply DS0)","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"DS0 is a 64 kbit/s digital voice channel from PCM; T1/DS1 carries 24 channels at 1.544 Mbit/s in North America; E1 carries 32 timeslots (30 voice) at 2.048 Mbit/s; time-division multiplexing onto trunks","domain":"technologyuk.net","publisherName":"TechnologyUK"},{"title":"Time for a big switch-up as PSTN switch-off looms","url":"https://www.openreach.com/news/time-for-a-big-switch-up-as-pstn-switch-off-looms/","excerpt":"BT Group will retire the PSTN network by 31st January 2027","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"BT Group will retire the PSTN by 31 January 2027; Openreach is withdrawing products that run over it and has placed exchanges under stop sell; migration to All-IP delivered as VoIP over broadband","domain":"openreach.com","publisherName":"Openreach"},{"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":"Analog network has reached end of serviceable life; industry-led migration to VoIP substantially complete by January 2027; Ofcom power-resilience requirement, including battery backup for emergency calls during a power cut","domain":"gov.uk","publisherName":"GOV.UK"}],"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":"ef14e83f-e61d-4263-80d2-b759e8ba8101","slug":"what-is-the-difference-between-pstn-and-pbx","question":"What is the difference between PSTN and PBX?","publishedAt":"2026-07-18T15:32:04.773","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":"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.","url":"/q/what-is-the-difference-between-pstn-and-pbx"}],"contributorStats":{"verifiedAnswers":224,"openDisputes":0},"schemaJson":{"@context":"https://schema.org","@type":"Question","name":"What is the PSTN (Public Switched Telephone Network)?","text":"What is the PSTN (Public Switched Telephone Network)?","url":"https://www.answerstack.io/q/what-is-the-pstn-public-switched-telephone-network","answerCount":1,"datePublished":"2026-07-18T15:17:01.345","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 public switched telephone network, or PSTN, is the aggregate of the world's interconnected, operator-run telephone networks, and it has carried ordinary landline calls for well over a century.[1] It works by circuit switching, meaning the network reserves a dedicated end-to-end path for each call and holds it open for the whole conversation, which is the main way it differs from the packet networks that carry internet traffic.[2] A call travels from your phone over a local loop to a local exchange, then across shared trunks and higher-level switches to the exchange serving the person you are calling, with a separate control network called SS7 setting up and releasing the connection.[1][4] Since the 1960s the core of the network has been digital, encoding each voice channel as a 64 kbit/s stream and multiplexing many of them onto shared circuits.[3][7] Carriers are now retiring the PSTN in favor of internet protocol networks, and in the United Kingdom the switch off is scheduled for 31 January 2027.[8][9]","url":"https://www.answerstack.io/q/what-is-the-pstn-public-switched-telephone-network","upvoteCount":0,"datePublished":"2026-07-18T15:17:01.345","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":"Public switched telephone network","url":"https://en.wikipedia.org/wiki/Public_switched_telephone_network"},{"@type":"CreativeWork","name":"PSTN (public switched telephone network)","url":"https://www.techtarget.com/searchnetworking/definition/PSTN"},{"@type":"CreativeWork","name":"ITU-T G.711: Pulse code modulation (PCM) of voice frequencies","url":"https://www.itu.int/rec/T-REC-G.711/"},{"@type":"CreativeWork","name":"ITU-T Q.700: Introduction to CCITT Signalling System No. 7","url":"https://www.itu.int/rec/T-REC-Q.700/"},{"@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":"About NANPA","url":"https://www.nanpa.com/about"},{"@type":"CreativeWork","name":"The Plesiochronous Digital Hierarchy","url":"https://www.technologyuk.net/telecommunications/communication-technologies/plesiochronous-digital-hierarchy.shtml"},{"@type":"CreativeWork","name":"Time for a big switch-up as PSTN switch-off looms","url":"https://www.openreach.com/news/time-for-a-big-switch-up-as-pstn-switch-off-looms/"},{"@type":"CreativeWork","name":"UK transition from analogue to digital landlines","url":"https://www.gov.uk/guidance/uk-transition-from-analogue-to-digital-landlines"}]}}}