The core difference is how each network moves a call. The public switched telephone network (PSTN) uses circuit switching, which reserves a dedicated path for the whole conversation, while Voice over Internet Protocol (VoIP) uses packet switching, breaking speech into data packets that share an IP network with your other traffic.[1][2][3] The PSTN runs over operator-owned copper and digital trunks and sets up calls with a separate signaling network called SS7, whereas VoIP runs over broadband and sets up calls with internet protocols such as SIP, carrying the audio in RTP packets.[1][5][6][7] Because VoIP treats voice as data, it usually costs less to run and can add features or capacity in software, though it now depends on your power and internet staying up.[2][9] The two are less rivals than successive generations: carriers are retiring the PSTN in favor of IP, and in the United Kingdom the switch off is scheduled for 31 January 2027.[2][10]
\n","summary":"PSTN and VoIP both carry phone calls, but they use opposite methods. The PSTN reserves a dedicated circuit for each call over operator-owned copper and digital trunks, while VoIP splits speech into packets that ride an ordinary internet connection. That single design choice drives the practical gaps in cost, features, mobility, reliability during a power cut, and emergency-call handling. Carriers are now retiring the PSTN and moving voice onto IP, with the UK PSTN switch off set for 31 January 2027.","publishedAt":"2026-07-18T15:31:59.18","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 VoIP","Difference between VoIP and PSTN","How is VoIP different from a landline?","Is VoIP the same as PSTN?","VoIP versus traditional phone line","Circuit switching vs packet switching phone calls","PSTN or VoIP: which is better?","What is the difference between a landline and VoIP?","Traditional phone network vs internet calling","Should I switch from PSTN to VoIP?","VoIP compared to the public switched telephone network","How does VoIP differ from 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":"b7a63a53-07a2-4362-a1c4-155832f643e5","sectionKey":"how_they_differ","sectionType":"markdown_section","heading":"How do PSTN and VoIP differ?","introMarkdown":"The difference is how each network carries a call. The public switched telephone network, usually shortened to PSTN, is the traditional telephone system, the aggregate of operator-run networks that has carried landline calls for more than a century using circuit switching.[1] Voice over Internet Protocol, or VoIP, carries calls as digital packets over an IP network such as your broadband connection.[2] Circuit switching sets up a dedicated path between two phones and holds it open for the entire conversation, so the capacity stays reserved even during the silences.[3] Packet switching works differently: it splits speech into small packets that share the same network as your email and web traffic, then reassembles them at the far end.[2] Almost every practical difference between the two, from cost and mobility to how a call behaves in a power cut, traces back to that one design choice.\n\nThe two also differ in what they run on and how they start a call. A PSTN call travels over copper lines and the operator's digital trunks, and a separate control network called Signalling System No. 7, or SS7, handles setting up and releasing each connection.[1][7] A VoIP call travels over the public internet or a private IP link, and internet protocols do the same job in software: the Session Initiation Protocol, or SIP, sets up and ends the call, while the audio rides in Real-time Transport Protocol packets.[5][6] Both networks digitize your voice in much the same way, often with the 64 kbit/s encoding defined in ITU-T G.711, so the raw sound is comparable; what differs is how each network carries those digits once they leave the phone.[4] It helps to see VoIP and the PSTN as successive generations rather than competitors, because carriers are now moving voice off the older network and onto IP, and where the two meet, gateways translate a call from one to the other.[2]","introHtml":"The difference is how each network carries a call. The public switched telephone network, usually shortened to PSTN, is the traditional telephone system, the aggregate of operator-run networks that has carried landline calls for more than a century using circuit switching.[1] Voice over Internet Protocol, or VoIP, carries calls as digital packets over an IP network such as your broadband connection.[2] Circuit switching sets up a dedicated path between two phones and holds it open for the entire conversation, so the capacity stays reserved even during the silences.[3] Packet switching works differently: it splits speech into small packets that share the same network as your email and web traffic, then reassembles them at the far end.[2] Almost every practical difference between the two, from cost and mobility to how a call behaves in a power cut, traces back to that one design choice.
\nThe two also differ in what they run on and how they start a call. A PSTN call travels over copper lines and the operator's digital trunks, and a separate control network called Signalling System No. 7, or SS7, handles setting up and releasing each connection.[1][7] A VoIP call travels over the public internet or a private IP link, and internet protocols do the same job in software: the Session Initiation Protocol, or SIP, sets up and ends the call, while the audio rides in Real-time Transport Protocol packets.[5][6] Both networks digitize your voice in much the same way, often with the 64 kbit/s encoding defined in ITU-T G.711, so the raw sound is comparable; what differs is how each network carries those digits once they leave the phone.[4] It helps to see VoIP and the PSTN as successive generations rather than competitors, because carriers are now moving voice off the older network and onto IP, and where the two meet, gateways translate a call from one to the other.[2]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":0},{"id":"74aeda8d-f7ed-446e-8a35-25e160bd3141","sectionKey":"comparison_table","sectionType":"table_section","heading":"PSTN and VoIP compared","introMarkdown":"The table sets the two networks side by side across the differences that matter most to a buyer. Each row gets its own section below, explaining what the difference means in practice and what to do about it.","introHtml":"The table sets the two networks side by side across the differences that matter most to a buyer. Each row gets its own section below, explaining what the difference means in practice and what to do about it.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{"rows":[{"cells":["How the call travels","Circuit switching over operator-owned copper and digital trunks; a path reserved for the whole call [1][3]","Packet switching over broadband or a private IP network; voice split into shared packets [2]"]},{"cells":["How a call is set up","SS7, a separate out-of-band signaling network [7]","SIP sets up the call, RTP carries the audio [5][6]"]},{"cells":["Cost and scaling","Physical lines; capacity added as hardware [1][2]","Lower infrastructure cost; capacity changed in software [2]"]},{"cells":["Power and outages","Powered from the exchange; often works in a blackout [11]","Needs local power and internet; goes down without backup [9]"]},{"cells":["Mobility","Number tied to a physical line and place [1]","Number travels with the user, device, or app [2]"]},{"cells":["Emergency calls","Tied to the fixed line's known address [1]","Provider must supply a validated or registered location; may be limited in an outage [9]"]}],"columns":["What differs","PSTN","VoIP"]},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":1},{"id":"32f3d264-4184-4fce-b0ab-cc4f69b24bbe","sectionKey":"circuit_vs_packet","sectionType":"markdown_section","heading":"How the call travels: circuit versus packet","introMarkdown":"The most basic difference is that the PSTN reserves a private circuit for each call while VoIP shares one network among all calls and data.[2][3] On the PSTN, dialing sets up a dedicated path from your phone, through the local exchange, across shared trunks, to the phone at the other end, and that path stays yours until someone hangs up.[1][3] The reserved capacity is guaranteed the moment the call connects, which is why a traditional line delivers such steady voice quality, though it also means the circuit sits idle and unavailable to anyone else during every pause in the conversation.[3] VoIP samples your voice, compresses it, and breaks it into packets that cross an IP network alongside everything else, finding their own route to the destination and reassembling there.[2] Sharing the network uses capacity far more efficiently, because nothing is reserved when no one is speaking, but it also means voice competes with other traffic, so a congested or unreliable connection shows up as choppy or delayed audio in a way a dedicated circuit never would.[2]","introHtml":"The most basic difference is that the PSTN reserves a private circuit for each call while VoIP shares one network among all calls and data.[2][3] On the PSTN, dialing sets up a dedicated path from your phone, through the local exchange, across shared trunks, to the phone at the other end, and that path stays yours until someone hangs up.[1][3] The reserved capacity is guaranteed the moment the call connects, which is why a traditional line delivers such steady voice quality, though it also means the circuit sits idle and unavailable to anyone else during every pause in the conversation.[3] VoIP samples your voice, compresses it, and breaks it into packets that cross an IP network alongside everything else, finding their own route to the destination and reassembling there.[2] Sharing the network uses capacity far more efficiently, because nothing is reserved when no one is speaking, but it also means voice competes with other traffic, so a congested or unreliable connection shows up as choppy or delayed audio in a way a dedicated circuit never would.[2]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":2},{"id":"da3cb79a-d21e-46ff-b259-48b55c2b7289","sectionKey":"call_setup","sectionType":"markdown_section","heading":"How each network sets up a call","introMarkdown":"The two networks use entirely different systems to start, route, and end a call. On the PSTN, a control network called Signalling System No. 7, standardized in the ITU-T Q.700 series, does this out of band, meaning it runs on its own channels rather than on the line carrying your voice.[7] SS7 passes the dialed number between switches, picks a route, rings the far end, and releases the circuit when the call is over, and it is also what makes caller ID and number portability possible.[1][7] VoIP moves that job into internet protocols. The Session Initiation Protocol, defined in IETF RFC 3261, locates the other party, negotiates the audio formats both sides support, and then connects or ends the call, while the speech travels in a separate stream using the Real-time Transport Protocol.[5][6] Both approaches still digitize your voice similarly, often with the 64 kbit/s pulse code modulation defined in ITU-T G.711, so the underlying audio can sound identical.[4] The practical result of the software approach is that adding a feature to VoIP usually means a configuration change, whereas a new PSTN feature historically meant altering the switches themselves.[1]","introHtml":"The two networks use entirely different systems to start, route, and end a call. On the PSTN, a control network called Signalling System No. 7, standardized in the ITU-T Q.700 series, does this out of band, meaning it runs on its own channels rather than on the line carrying your voice.[7] SS7 passes the dialed number between switches, picks a route, rings the far end, and releases the circuit when the call is over, and it is also what makes caller ID and number portability possible.[1][7] VoIP moves that job into internet protocols. The Session Initiation Protocol, defined in IETF RFC 3261, locates the other party, negotiates the audio formats both sides support, and then connects or ends the call, while the speech travels in a separate stream using the Real-time Transport Protocol.[5][6] Both approaches still digitize your voice similarly, often with the 64 kbit/s pulse code modulation defined in ITU-T G.711, so the underlying audio can sound identical.[4] The practical result of the software approach is that adding a feature to VoIP usually means a configuration change, whereas a new PSTN feature historically meant altering the switches themselves.[1]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":3},{"id":"271d2bc3-e8a0-4596-94dc-7b785cc7b00d","sectionKey":"cost_scaling","sectionType":"markdown_section","heading":"Cost and how capacity scales","introMarkdown":"VoIP is generally cheaper to run and quicker to scale than the PSTN, mostly because it removes the dedicated physical lines.[2] On the PSTN, capacity is a matter of hardware: each conversation needs its own circuit, and adding lines or long-distance capacity means provisioning more physical connections.[1][2] Because VoIP treats a call as data on a network you already have, providers report lower infrastructure costs, and you can add or remove call capacity in software rather than by ordering new lines.[2] That software model is also why VoIP carries features a circuit-switched line cannot, such as app integrations and answering the work number from any device signed into the account.[2][12] The savings are not automatic. VoIP shifts cost onto your internet connection and network equipment, so a business moving off the PSTN usually has to invest in reliable bandwidth and backup power to hold call quality and availability where a dedicated line kept them by default.[2] The fair comparison is total cost across both the phone service and the network it now depends on, not the monthly line charge alone.","introHtml":"VoIP is generally cheaper to run and quicker to scale than the PSTN, mostly because it removes the dedicated physical lines.[2] On the PSTN, capacity is a matter of hardware: each conversation needs its own circuit, and adding lines or long-distance capacity means provisioning more physical connections.[1][2] Because VoIP treats a call as data on a network you already have, providers report lower infrastructure costs, and you can add or remove call capacity in software rather than by ordering new lines.[2] That software model is also why VoIP carries features a circuit-switched line cannot, such as app integrations and answering the work number from any device signed into the account.[2][12] The savings are not automatic. VoIP shifts cost onto your internet connection and network equipment, so a business moving off the PSTN usually has to invest in reliable bandwidth and backup power to hold call quality and availability where a dedicated line kept them by default.[2] The fair comparison is total cost across both the phone service and the network it now depends on, not the monthly line charge alone.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":4},{"id":"cb6ef342-186b-4855-a96f-e1dbac93667c","sectionKey":"power_quality","sectionType":"markdown_section","heading":"Power, outages, and call quality","introMarkdown":"A traditional PSTN line usually keeps working in a power cut, while a VoIP phone stops unless you plan for backup power.[9][11] That resilience is a large part of why regulators treat the switch to digital voice as a safety matter rather than a convenience. VoIP relies on your local power and your internet link, so when the electricity fails, the router and phones go down unless a battery keeps them running.[9] In the United Kingdom, providers moving customers to digital lines must offer a battery backup unit so people can still reach emergency services during a power cut, and in the United States, interconnected VoIP providers have to warn subscribers plainly that 911 may be unavailable during a loss of power or a broadband failure.[9][11] Call quality also behaves differently. The PSTN's reserved circuit gives steady audio by design, whereas VoIP quality depends on the network beneath it, so delay, jitter, and packet loss can degrade a call that a dedicated line would have carried cleanly.[2][3] Prioritizing voice traffic and keeping spare bandwidth is what closes most of that gap.","introHtml":"A traditional PSTN line usually keeps working in a power cut, while a VoIP phone stops unless you plan for backup power.[9][11] That resilience is a large part of why regulators treat the switch to digital voice as a safety matter rather than a convenience. VoIP relies on your local power and your internet link, so when the electricity fails, the router and phones go down unless a battery keeps them running.[9] In the United Kingdom, providers moving customers to digital lines must offer a battery backup unit so people can still reach emergency services during a power cut, and in the United States, interconnected VoIP providers have to warn subscribers plainly that 911 may be unavailable during a loss of power or a broadband failure.[9][11] Call quality also behaves differently. The PSTN's reserved circuit gives steady audio by design, whereas VoIP quality depends on the network beneath it, so delay, jitter, and packet loss can degrade a call that a dedicated line would have carried cleanly.[2][3] Prioritizing voice traffic and keeping spare bandwidth is what closes most of that gap.
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":5},{"id":"aedb8e17-717e-418f-97de-01b05ceedf74","sectionKey":"mobility","sectionType":"markdown_section","heading":"Mobility and where a phone number lives","introMarkdown":"A PSTN number is tied to a physical line at a fixed address, whereas a VoIP number travels with the user.[1][2] On the PSTN, your number belongs to the copper pair running to a specific building, so the service and the location are effectively the same thing, and a call could only reach you where that line physically ended.[1] VoIP separates the number from any place. Because the account lives on an IP network, one number can ring a desk phone, a laptop app, or a mobile, and it follows the user from the office to home to anywhere with a connection.[2] For distributed and hybrid teams, that portability is one of the main reasons to move off the PSTN, since it lets someone keep a single work number across every device.[2] The same freedom creates a complication the fixed line never had. When a number is not tied to a place, the network does not automatically know where a call is coming from, which is why emergency calling has to be handled deliberately for VoIP in a way the PSTN never required.[9]","introHtml":"A PSTN number is tied to a physical line at a fixed address, whereas a VoIP number travels with the user.[1][2] On the PSTN, your number belongs to the copper pair running to a specific building, so the service and the location are effectively the same thing, and a call could only reach you where that line physically ended.[1] VoIP separates the number from any place. Because the account lives on an IP network, one number can ring a desk phone, a laptop app, or a mobile, and it follows the user from the office to home to anywhere with a connection.[2] For distributed and hybrid teams, that portability is one of the main reasons to move off the PSTN, since it lets someone keep a single work number across every device.[2] The same freedom creates a complication the fixed line never had. When a number is not tied to a place, the network does not automatically know where a call is coming from, which is why emergency calling has to be handled deliberately for VoIP in a way the PSTN never required.[9]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":6},{"id":"5a23ab12-ddd8-4a33-801f-a3aa180894cb","sectionKey":"emergency_regulation","sectionType":"markdown_section","heading":"Emergency calls and how each is regulated","introMarkdown":"Both networks reach emergency services, but VoIP has to work harder to tell responders where you are, and regulators have written specific rules to make sure it does.[8][9] A PSTN call comes from a fixed line, so the address on file for that line tells the emergency center where to send help.[1] A VoIP call can originate from anywhere, so the location has to be supplied another way. In the United States, a service that lets you both place and receive calls to the regular phone network is defined as interconnected VoIP, and that category carries duties resembling a phone company's.[8] Those providers must route 911 calls to the right center with a call-back number and a usable location, and they must tell customers plainly where 911 might not work, such as during a power or internet outage.[9] The location can be a validated street address for a fixed service or an address the user registers for a service that moves, which puts some responsibility on you to keep that address current.[9] An app that only calls other users of the same app is not held to these emergency rules, because it never connects to the public phone network.[8]","introHtml":"Both networks reach emergency services, but VoIP has to work harder to tell responders where you are, and regulators have written specific rules to make sure it does.[8][9] A PSTN call comes from a fixed line, so the address on file for that line tells the emergency center where to send help.[1] A VoIP call can originate from anywhere, so the location has to be supplied another way. In the United States, a service that lets you both place and receive calls to the regular phone network is defined as interconnected VoIP, and that category carries duties resembling a phone company's.[8] Those providers must route 911 calls to the right center with a call-back number and a usable location, and they must tell customers plainly where 911 might not work, such as during a power or internet outage.[9] The location can be a validated street address for a fixed service or an address the user registers for a service that moves, which puts some responsibility on you to keep that address current.[9] An app that only calls other users of the same app is not held to these emergency rules, because it never connects to the public phone network.[8]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":7},{"id":"b4dbbcd5-b17a-4b23-9038-4dce6ae67af0","sectionKey":"how_they_work_together","sectionType":"markdown_section","heading":"How PSTN and VoIP work together","introMarkdown":"The PSTN and VoIP connect to each other constantly, so a call rarely has to be purely one or the other.[1][2] Where the two networks meet, a gateway translates a call from packets to a circuit or back, which is how a VoIP phone can ring an ordinary landline and how a landline can reach a VoIP number.[1] Most calls now cross both worlds at some point, especially while carriers are partway through retiring the older network.[2] Choosing between them is less about which technology is better in the abstract and more about your situation. A phone system that already works, sits at one location, and rarely changes can run on a traditional line until the carrier retires it. An organization with remote staff, several sites, or a need to bring calling into the tools a team already uses tends to fit VoIP better, because VoIP was built for that flexibility.[2] The timing is increasingly set by the carrier anyway. Since operators are moving voice onto IP to avoid running two networks, the practical question for most organizations is one of timing and preparation: when to move, and how to cover the power and location gaps before the PSTN underneath them is switched off.[2][10]","introHtml":"The PSTN and VoIP connect to each other constantly, so a call rarely has to be purely one or the other.[1][2] Where the two networks meet, a gateway translates a call from packets to a circuit or back, which is how a VoIP phone can ring an ordinary landline and how a landline can reach a VoIP number.[1] Most calls now cross both worlds at some point, especially while carriers are partway through retiring the older network.[2] Choosing between them is less about which technology is better in the abstract and more about your situation. A phone system that already works, sits at one location, and rarely changes can run on a traditional line until the carrier retires it. An organization with remote staff, several sites, or a need to bring calling into the tools a team already uses tends to fit VoIP better, because VoIP was built for that flexibility.[2] The timing is increasingly set by the carrier anyway. Since operators are moving voice onto IP to avoid running two networks, the practical question for most organizations is one of timing and preparation: when to move, and how to cover the power and location gaps before the PSTN underneath them is switched off.[2][10]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":8},{"id":"ab85798d-ff41-492e-bb20-56f601d4a0d8","sectionKey":"what_it_is_not","sectionType":"markdown_section","heading":"What the difference is not","introMarkdown":"The PSTN versus VoIP distinction is about how calls are carried, not about several things people often assume it means.\n\n### It is not the same as landline versus mobile\n\nA landline can be VoIP, and a mobile call is often carried as IP inside the cellular network, so the neat pairing of landline with PSTN and mobile with VoIP breaks down.[2] Many home landlines in the United Kingdom already run over broadband as VoIP while looking and working exactly like the old service.[10]\n\n### VoIP is not a single app or brand\n\nVoIP is a category, not a product. It covers consumer calling apps, desk phones, carrier trunks, and full business phone systems, so two VoIP services can share none of the same software and still both be VoIP.[2]\n\n### A switch off does not mean copper vanishes overnight\n\nRetiring the PSTN is about replacing the circuit-switched way of connecting calls, not pulling out every copper wire on a single date. In the United Kingdom the target is 31 January 2027, while in the United States the move from copper and legacy equipment to IP is running on a longer, less fixed timeline.[2][10]\n\n### Neither one guarantees better sound\n\nBoth networks can carry the same 64 kbit/s voice encoding, so VoIP is not automatically clearer and the PSTN is not automatically more reliable.[4] Sound quality on VoIP depends on the network it rides, so modern wideband VoIP can beat a traditional line while a congested connection can fall below it.[2]","introHtml":"The PSTN versus VoIP distinction is about how calls are carried, not about several things people often assume it means.
\nA landline can be VoIP, and a mobile call is often carried as IP inside the cellular network, so the neat pairing of landline with PSTN and mobile with VoIP breaks down.[2] Many home landlines in the United Kingdom already run over broadband as VoIP while looking and working exactly like the old service.[10]
\nVoIP is a category, not a product. It covers consumer calling apps, desk phones, carrier trunks, and full business phone systems, so two VoIP services can share none of the same software and still both be VoIP.[2]
\nRetiring the PSTN is about replacing the circuit-switched way of connecting calls, not pulling out every copper wire on a single date. In the United Kingdom the target is 31 January 2027, while in the United States the move from copper and legacy equipment to IP is running on a longer, less fixed timeline.[2][10]
\nBoth networks can carry the same 64 kbit/s voice encoding, so VoIP is not automatically clearer and the PSTN is not automatically more reliable.[4] Sound quality on VoIP depends on the network it rides, so modern wideband VoIP can beat a traditional line while a congested connection can fall below it.[2]
\n","outroMarkdown":null,"outroHtml":null,"contentJson":{},"configJson":{},"noteMarkdown":null,"noteHtml":null,"sortOrder":9},{"id":"3c8b51c6-fba8-41f1-b58c-51ead1b62b0c","sectionKey":"contributor_perspective","sectionType":"markdown_section","heading":"How this answer was researched","introMarkdown":"This entry keeps the technical claims, which come from the standards themselves, separate from the regulatory and market claims, which come from the rules and the bodies managing the transition. The circuit-switching and packet-switching definitions are drawn from reference sources and cross-checked, and the signaling and codec descriptions come from the ITU-T and IETF documents that specify SS7, SIP, RTP, and G.711.[1][2][3][4][5][6][7] The interconnected-VoIP and 911 rules are taken from the Code of Federal Regulations rather than secondary summaries.[8][9] The retirement timeline reflects the network operator and the UK government guidance that set the 31 January 2027 date and the power-resilience requirement.[10][11] One industry source is used only to corroborate the general description of VoIP as a packet-based alternative to the PSTN, never as sole support for a claim.[12] Telecom standards and retirement schedules change, so the dates and figures 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 keeps the technical claims, which come from the standards themselves, separate from the regulatory and market claims, which come from the rules and the bodies managing the transition. The circuit-switching and packet-switching definitions are drawn from reference sources and cross-checked, and the signaling and codec descriptions come from the ITU-T and IETF documents that specify SS7, SIP, RTP, and G.711.[1][2][3][4][5][6][7] The interconnected-VoIP and 911 rules are taken from the Code of Federal Regulations rather than secondary summaries.[8][9] The retirement timeline reflects the network operator and the UK government guidance that set the 31 January 2027 date and the power-resilience requirement.[10][11] One industry source is used only to corroborate the general description of VoIP as a packet-based alternative to the PSTN, never as sole support for a claim.[12] Telecom standards and retirement schedules change, so the dates and figures 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":10}],"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; copper local lines and digital trunks; SS7 controlling calls; digital core with G.711 and 64 kbit/s channels; operators transitioning telephony to Voice over IP","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; VoIP turns audio into data packets over an IP network; VoIP advantages of lower infrastructure cost, scalability, and advanced features such as UC and app integrations; providers transitioning from PSTN to all-IP","domain":"techtarget.com","publisherName":"Informa TechTarget"},{"title":"Circuit switching","url":"https://en.wikipedia.org/wiki/Circuit_switching","excerpt":"The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the communication session.","quoteText":null,"sourceRole":"INDEPENDENT","verifiedAt":"2026-07-18T00:00:00","supportsText":"Circuit switching establishes a dedicated channel that guarantees full bandwidth for the duration of the session and stays reserved even during silence; packet switching shares links among competing sessions","domain":"en.wikipedia.org","publisherName":"Wikipedia"},{"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 of voice frequencies, the 64 kbit/s baseline digital voice encoding shared by the PSTN core and VoIP; in force","domain":"itu.int","publisherName":"International Telecommunication Union (ITU-T)"},{"title":"RFC 3261: SIP: Session Initiation Protocol","url":"https://datatracker.ietf.org/doc/html/rfc3261","excerpt":"an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"SIP is the application-layer signaling protocol that creates, modifies, and ends VoIP sessions such as internet telephone calls","domain":"datatracker.ietf.org","publisherName":"IETF"},{"title":"RFC 3550: RTP: A Transport Protocol for Real-Time Applications","url":"https://datatracker.ietf.org/doc/html/rfc3550","excerpt":"RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"RTP carries the real-time audio media of a VoIP call as a separate stream from signaling","domain":"datatracker.ietf.org","publisherName":"IETF"},{"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. Status: In force.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"Signalling System No. 7 (SS7) is the PSTN out-of-band signaling system that sets up, routes, and releases calls; standardized in the Q.700 series; in force","domain":"itu.int","publisherName":"International Telecommunication Union (ITU-T)"},{"title":"47 CFR 9.3, Definitions: Interconnected VoIP service","url":"https://www.law.cornell.edu/cfr/text/47/9.3","excerpt":"Permits users generally to receive calls that originate on the public switched telephone network and to terminate calls to the public switched telephone network.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"Regulatory definition of interconnected VoIP: real-time two-way voice, broadband, IP-compatible equipment, and calls received from and terminated to the PSTN; app-to-app-only services fall outside it","domain":"law.cornell.edu","publisherName":"Legal Information Institute, Cornell Law School"},{"title":"47 CFR 9.11: E911 service requirements for interconnected VoIP","url":"https://www.law.cornell.edu/cfr/text/47/9.11","excerpt":"of the circumstances under which E911 service may not be available through the interconnected VoIP service or may be in some way limited.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"Interconnected VoIP E911 duties: transmit 911 calls with call-back number and location, and warn subscribers plainly that E911 may be unavailable or limited during loss of electrical power or broadband connection failure","domain":"law.cornell.edu","publisherName":"Legal Information Institute, Cornell Law School"},{"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; landline calls move to All-IP delivered as Voice over IP 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":"Your landline provider should offer you a solution such as a battery back-up unit. This will mean that you can still make emergency calls during a power cut.","quoteText":null,"sourceRole":"PRIMARY","verifiedAt":"2026-07-18T00:00:00","supportsText":"Analogue network has reached the end of its serviceable life; migration to VoIP and digital voice; Ofcom power-resilience requirement including a battery backup unit so customers can still make emergency calls during a power cut","domain":"gov.uk","publisherName":"GOV.UK"},{"title":"Business VoIP Service: 6 Things You Need to Know","url":"https://www.atlantech.net/blog/business-voip-service-6-things-you-need-to-know","excerpt":"VoIP services use data connectivity for the transmission of voice packets. VoIP is an alternative to the public switched telephone network (PSTN).","quoteText":null,"sourceRole":"SUPPORTING","verifiedAt":"2026-07-18T00:00:00","supportsText":"General description of VoIP as an IP-based, data-packet phone service that is an alternative to the PSTN; corroborated by [2] and [1]","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":"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 difference between PSTN and VoIP?","text":"What is the difference between PSTN and VoIP?","url":"https://www.answerstack.io/q/what-is-the-difference-between-pstn-and-voip","answerCount":1,"datePublished":"2026-07-18T15:31:59.18","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 core difference is how each network moves a call. The public switched telephone network (PSTN) uses circuit switching, which reserves a dedicated path for the whole conversation, while Voice over Internet Protocol (VoIP) uses packet switching, breaking speech into data packets that share an IP network with your other traffic.[1][2][3] The PSTN runs over operator-owned copper and digital trunks and sets up calls with a separate signaling network called SS7, whereas VoIP runs over broadband and sets up calls with internet protocols such as SIP, carrying the audio in RTP packets.[1][5][6][7] Because VoIP treats voice as data, it usually costs less to run and can add features or capacity in software, though it now depends on your power and internet staying up.[2][9] The two are less rivals than successive generations: carriers are retiring the PSTN in favor of IP, and in the United Kingdom the switch off is scheduled for 31 January 2027.[2][10]","url":"https://www.answerstack.io/q/what-is-the-difference-between-pstn-and-voip","upvoteCount":0,"datePublished":"2026-07-18T15:31:59.18","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":"Circuit switching","url":"https://en.wikipedia.org/wiki/Circuit_switching"},{"@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":"RFC 3261: SIP: Session Initiation Protocol","url":"https://datatracker.ietf.org/doc/html/rfc3261"},{"@type":"CreativeWork","name":"RFC 3550: RTP: A Transport Protocol for Real-Time Applications","url":"https://datatracker.ietf.org/doc/html/rfc3550"},{"@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":"47 CFR 9.3, Definitions: Interconnected VoIP service","url":"https://www.law.cornell.edu/cfr/text/47/9.3"},{"@type":"CreativeWork","name":"47 CFR 9.11: E911 service requirements for interconnected VoIP","url":"https://www.law.cornell.edu/cfr/text/47/9.11"},{"@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"},{"@type":"CreativeWork","name":"Business VoIP Service: 6 Things You Need to Know","url":"https://www.atlantech.net/blog/business-voip-service-6-things-you-need-to-know"}]}}}