The PSTN, or public switched telephone network, is the interconnected set of telephone networks that carries calls between public telephone numbers. It is defined less by any one technology than by three things it provides: a global numbering plan, switching that connects any number to any other number, and interconnection agreements between operators so that a call can cross from one network to another. Traditionally it did this with circuit switching over copper and fibre, giving each call its own dedicated channel for as long as it lasted.
Two clarifications matter straight away. First, in strict usage the PSTN is the fixed telephone network, while mobile networks are public land mobile networks that interconnect with it. In everyday supplier language, “PSTN termination” simply means the ability to reach any public telephone number, fixed or mobile. Second, the underlying technology is being replaced. Operators are retiring circuit-switched equipment and carrying the same calls over IP, which changes the plumbing without changing the numbering plan or the obligations attached to it.
Quick facts
| Question | Short answer |
|---|---|
| What does PSTN stand for? | Public switched telephone network |
| What defines it? | Numbering, switching between any two numbers, and operator interconnection |
| How were calls carried? | Circuit switching, with a channel reserved for the whole call |
| What is the numbering standard? | ITU-T Recommendation E.164, whose current edition is dated February 2026 |
| What carried the audio? | A 64 kbit/s channel, coded per ITU-T G.711 |
| What carried the call setup? | Signalling System No. 7, specified in the ITU-T Q.700 series |
| Is it being switched off? | The legacy equipment is being retired country by country. Phone numbers are not going away |
| Does VoIP replace it? | VoIP replaces the transport. Calls to public numbers still interconnect with the same network of operators |
What actually makes a network “the PSTN”
Strip away the copper and four properties remain.
A shared numbering plan. Every reachable number fits the international numbering plan in ITU-T E.164, with a country code followed by national digits and a limit of 15 digits in total. This is what makes a number dialled in one country meaningful in another.
Switching, not point to point wiring. The word “switched” is the important one. Rather than a wire between every pair of subscribers, exchanges connect calls on demand, which is why a network of a few thousand exchanges can connect billions of endpoints.
Interconnection between operators. No single company owns the telephone network. It works because operators agree to hand calls to each other, settle payments for doing so, and follow common signalling rules.
Regulatory obligations attached to numbers. Emergency access, number portability and lawful requirements travel with the number, not with the technology carrying it. This is the part people most often forget when they assume an IP service is a private matter between vendor and customer.
How a PSTN call is set up and carried
A traditional call runs on two planes that stay separate.
The signalling plane carries the call control. When a number is dialled, the originating exchange sends signalling messages that identify the destination, ask the next network to reserve resources, and report answer and release. On the international network this is Signalling System No. 7, whose ISDN User Part handles ordinary call setup. Keeping signalling out of the audio path is what allows a call to be routed, screened and billed before any sound flows.
The media plane carries the audio. Once a path is agreed, a channel is held between the endpoints for the whole call. It stays reserved through silence, through hold, and through the part where nobody says anything.
The route usually passes through a local exchange serving the caller’s number range, one or more transit exchanges, and, where the call leaves the country or crosses to a mobile network, a gateway. At each hop the dialled number is the routing key.
That reserved channel explains most of the old network’s personality: predictable quality, high cost per simultaneous call, capacity bought in fixed blocks, and long lead times to add lines.
Why phone calls sound the way they do
The classic voice channel is 64 kbit/s, using the pulse code modulation defined in ITU-T G.711. It carries a narrow slice of the audio spectrum, roughly the range that keeps speech intelligible, which is why a traditional call sounds thinner than a voice note recorded on the same phone.
This was a deliberate engineering trade in the 1960s and 1970s: enough quality to recognise a voice and understand words, at a bit rate that let one trunk carry many conversations. The consequence is that even today, when a call touches a legacy interconnect, wideband audio is lost at that point regardless of what the endpoints support.
PSTN, PLMN and VoIP: what each term covers
| Term | What it means | Where the confusion starts |
|---|---|---|
| PSTN | The public fixed telephone network reachable by E.164 numbers | Used loosely to mean any public phone number, including mobile |
| PLMN | A public land mobile network, the operator network serving mobile subscriptions | Mobile networks interconnect with the fixed network. 3GPP describes the mobile switching centre as the functional interface between fixed networks and a mobile network for circuit-switched call setup, in 3GPP TS 23.002 |
| VoIP | Carrying voice over IP networks | VoIP is a transport choice. A VoIP service that uses public numbers is part of the same interconnected phone system |
| SIP trunk | A signalling and media connection between a customer’s phone system and a carrier | It replaces the ISDN circuit, not the number |
The useful mental model: the PSTN is a numbering and interconnection system that used to imply a specific transport. The transport has changed. The system has not.
Where the PSTN and SIP networks meet
Almost every call today crosses this boundary at least once. Two things have to be translated.
Signalling. SIP has to be mapped to and from ISUP so that setup, answer, release and cause codes survive. That interworking is specified in ITU-T Q.1912.5, which defines how SIP and ISUP or BICC exchange call control across a network to network interface. It is also why some failure causes on IP calls read like circuit-switched ones.
Media. The audio has to move between IP packets and whatever the legacy side expects, with transcoding if the codecs differ. This costs a little delay and, when narrowband is involved anywhere on the path, caps the quality for everyone on the call.
For a business, the practical consequence is that a modern voice platform never needs to speak SS7. It speaks SIP to a carrier, and the carrier owns the translation, the interconnects and the regulatory relationships behind them.
What replaces the PSTN, piece by piece
The retirement programmes now under way are not removing telephony. They are removing a generation of equipment. Each part has a direct equivalent: the access line becomes broadband with a SIP trunk or a hosted service, the local exchange becomes software, trunk circuits become IP transport, ISUP becomes SIP, and the fixed 64 kbit/s channel becomes a codec negotiated per call.
Two things survive untouched. Numbers keep working, and the obligations attached to them keep applying.
The switch-off, accurately stated
Different countries are doing this differently, and the detail is frequently misreported.
United Kingdom. Landline services move to a fully digital network by January 2027, with the PSTN withdrawn. The House of Commons Library briefing is explicit that the withdrawal is industry-led rather than the result of government policy, and that migration decisions sit with the companies operating the network. The original industry target was 31 December 2025, and the programme was paused twice over concerns about customers who depend on landlines and connected devices such as telecare alarms. Migration is well advanced: the briefing cites Ofcom figures showing PSTN landline customers falling from 5.2 million in July 2024 to 3.2 million in July 2025.
United States. There is no single national date. Copper retirement and the move off time division multiplexing happen carrier by carrier under regulatory supervision, and the framework itself is being loosened to speed things up. In July 2025 the FCC opened a rulemaking on copper retirement proposing revisions to its network change disclosure rules and to the section 214(a) discontinuance process.
Elsewhere. Timelines vary widely by market, and some countries completed their migrations years ago. Treat any single global date you see as marketing rather than fact.
What changes for a business when the local network goes IP
- Power resilience becomes your problem. A traditional analogue line drew power from the exchange. A digital line depends on equipment in the building. In the UK, Ofcom requires providers to ensure uninterrupted access to emergency services for at least an hour, including during a power cut, which is a floor rather than a plan.
- Anything else on the line needs checking. Lift emergency phones, door entry systems, alarm diallers, telecare devices, payment terminals and fax machines were all built on the assumption of an analogue pair.
- ISDN capacity thinking stops applying. Concurrent call limits become a commercial setting rather than a physical count of channels, which changes how you plan for peaks.
- Numbers move with you. Porting is the mechanism that keeps published numbers working when the underlying service changes provider or technology.
- Audio quality becomes a design choice. Codec selection and route quality now vary per call rather than being fixed by the network.
Common misconceptions
“The PSTN switch-off means phone numbers are being retired.” No. Numbering plans, portability and emergency obligations continue. Only the equipment changes.
“The PSTN is government-run infrastructure.” It is a set of commercially operated, interconnected networks working to common standards under regulatory supervision. In the UK the retirement programme is industry-led.
“VoIP is separate from the phone network.” A VoIP service that originates or terminates calls on public numbers is interconnected with the same system. The transport differs, the numbering does not.
“PSTN means landlines, so mobile is unrelated.” Mobile networks are distinct networks that interconnect with the fixed network. In everyday commercial usage, PSTN reachability covers both.
“IP calls are always better quality.” They can be, and they can also be worse. Quality now depends on the route, the codec and the network path rather than being fixed by a reserved channel.
Where didlogic fits
didlogic operates at the interconnection layer of this picture. Customer platforms speak SIP, and didlogic handles the carrier side: delivering inbound calls to DID and virtual numbers and passing outbound calls to the networks that serve the destination through international voice termination.
That division of labour is the practical answer to “how do I connect to the PSTN”. A business does not interconnect with the public telephone network directly. It connects to a carrier that already holds the numbering resources, the interconnects and the regulatory permissions in each market, and reaches everything else through them. Existing numbers can be brought across through number porting so that published contact details survive a change of technology, which is the same problem the switch-off programmes are solving at national scale.
What happens after the call arrives, such as menus, queues, routing rules or voice applications, belongs to the customer’s own platform.
FAQs
Is the PSTN the same as a landline?
A landline is one service delivered over the PSTN. The network itself is the wider system of numbering, switching and interconnection that connects public telephone numbers to each other.
What is the difference between the PSTN and VoIP?
VoIP describes carrying voice over IP networks. The PSTN describes the public numbered telephone system. A VoIP service that makes and receives calls on public numbers interconnects with that system rather than replacing it.
Will my phone number stop working when the PSTN is switched off?
No. Migration moves the service to a digital connection and keeps the number. Numbering plans and portability rules continue to apply.
Why do PSTN calls sound narrowband?
Because the traditional voice channel carries 64 kbit/s of narrowband audio under ITU-T G.711. When any leg of a call crosses that kind of interconnect, the call is limited to it.
Does an IP phone line work in a power cut?
Not by itself. It depends on powered equipment at the premises, which is why regulators set requirements around continued access to emergency services and why battery backup is part of migration planning.
What is SS7 and is it going away?
Signalling System No. 7 is the signalling system that sets up and releases calls between operators, specified in the ITU-T Q.700 series. As networks move to IP, SIP takes over that role, with interworking defined so the two can coexist during the long transition.
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