Public switched telephone network (PSTN)
Definition
Public switched telephone network (PSTN)
The public switched telephone network (PSTN) is the worldwide collection of circuit-switched telephone systems that carry voice calls between fixed lines, mobile networks, and satellite links. Built on copper, fibre, and microwave gear since the 1880s, it still routes a sizeable share of regulated emergency and business voice traffic today.
Key takeaways
- PSTN uses dedicated circuits between two endpoints, so call quality stays predictable even when the public internet is congested.
- Carriers are sunsetting copper PSTN by the early 2030s in the US, UK, and most of the EU, pushing voice onto IP networks.
- Contact centres in the Philippines and India still rely on PSTN trunks for inbound toll-free numbers and emergency-line compliance.
The PSTN started life as Alexander Graham Bell’s local exchanges and grew into a globally interconnected mesh of national telcos. Today it sits alongside Voice over Internet Protocol (VoIP) and mobile data, but the underlying circuit-switched logic still anchors phone numbering, call routing, and lawful-intercept rules.
For outsourcing buyers, the PSTN matters because regulators in many markets still require contact centres to terminate certain inbound calls (000, 911, 999, 112) over circuit-switched lines.
How it works
The PSTN sets up a dedicated electrical or optical path between two phones for the length of a call, using a hierarchy of local exchanges, tandem switches, and international gateways. Signalling System 7 (SS7) tells those switches where to send the call, while E.164 numbering keeps every line uniquely addressable across borders.
A typical call flow looks like this:
- You lift the handset and the local exchange detects the off-hook signal.
- You dial; SS7 messages route the request through tandem or transit switches.
- The far-end exchange rings the destination phone and reserves a 64 kbps circuit.
- The circuit stays open end-to-end until one party hangs up.
That fixed circuit is why PSTN voice sounds consistent — no jitter, no buffering, no packet loss. It’s also why the system can’t multiplex easily; one call holds one channel for its full duration.
| Layer | Job | Example kit |
|---|---|---|
| Access | Connects the phone to the local exchange | Copper twisted pair, ISDN, fibre to the cabinet |
| Switching | Routes calls between exchanges | Class 5 local switch, Class 4 tandem |
| Signalling | Sets up, tears down, and bills calls | SS7, ISUP, C7 |
| Transmission | Carries the call between switches | Microwave, fibre, undersea cable |
The International Telecommunication Union sets the numbering, signalling, and quality standards that let the world’s national PSTNs interoperate. National regulators — the FCC in the US and Ofcom in the UK — then translate those standards into licence conditions.
Examples
BT Openreach (UK). BT has set a December 2027 deadline to switch off the legacy PSTN and migrate every UK landline to Digital Voice over IP. Ofcom confirmed the migration timeline in 2023 after pausing it for vulnerable-user concerns.
AT&T (United States). AT&T petitioned the FCC to retire copper PSTN across most of its 21-state footprint by 2029, replacing it with fibre and fixed wireless. The FCC granted partial waivers in 2024.
Globe Telecom and PLDT (Philippines). Both carriers still run a substantial PSTN footprint for business process outsourcing clients in Manila and Cebu. Toll-free 1-800 numbers terminated in the Philippines remain anchored to PSTN trunks for inbound contact-centre traffic.
Deutsche Telekom (Germany). Completed an all-IP migration in 2018, six years ahead of most European peers. Voice now runs over IMS, not the legacy circuit-switched core, though emergency calls still touch PSTN gateways.
Related terms
- Voice over Internet Protocol (VoIP) is the IP-based successor that carries voice as data packets.
- Contact centre operations route inbound and outbound calls across PSTN and VoIP trunks.
- Business process outsourcing firms buy PSTN and SIP capacity to serve their voice clients.
- Interactive voice response sits between the PSTN trunk and a live agent.
- Toll-free number routing still depends on PSTN translation databases.
- Session initiation protocol signals modern voice calls the way SS7 once did for PSTN.
- Automatic call distributor hardware historically connected directly to PSTN trunks.
FAQ
Is the PSTN being shut down?
Yes, in stages. The UK targets December 2027, Germany finished in 2018, and the US is on a rolling state-by-state retirement schedule with FCC oversight. Most developing markets will keep PSTN running into the 2030s.
What’s the difference between PSTN and VoIP?
PSTN reserves a dedicated 64 kbps circuit per call over copper or fibre. VoIP breaks voice into packets and sends them over the public internet or a private IP network. PSTN is more predictable; VoIP is cheaper and more flexible.
Why do contact centres still use PSTN?
Regulators in most countries require emergency and some toll-free numbers to terminate on circuit-switched lines. Many enterprise voice contracts also pre-date the IP migration, so call centres keep mixed PSTN-and-SIP trunks until renewal.
How fast is a PSTN line?
A single voice channel carries 64 kbps of digitised audio. Data calls over dial-up modems peaked at 56 kbps. ISDN lines can bond two 64 kbps channels for 128 kbps. None of those numbers compete with broadband.
Does PSTN work during a power cut?
Traditional copper PSTN delivers line power from the exchange, so a corded handset rings during a local outage. Fibre-replacement Digital Voice phones lose power with the home and need a battery backup, which is why Ofcom mandated free backup units for vulnerable UK customers.
Outsource Accelerator helps growing companies match with vetted contact-centre partners across the Philippines, India, and South Africa — browse 4,000+ verified BPO firms to find the right voice partner.
Independent
