What is the difference between PSTN and VoIP?

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.

The 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]

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]

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]

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.

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.

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]

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]

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]

The PSTN versus VoIP distinction is about how calls are carried, not about several things people often assume it means.

It is not the same as landline versus mobile

A 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]

VoIP is not a single app or brand

VoIP 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]

A switch off does not mean copper vanishes overnight

Retiring 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]

Neither one guarantees better sound

Both 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]