Public switched telephone network (PSTN)
Definition
Public switched telephone network (PSTN)
The public switched telephone network (PSTN) is the mesh of circuit-switched links that carry voice between fixed lines, mobile networks and satellite. Built on copper and fibre since the 1880s, it is still the line of last resort for regulated emergency calls.
The PSTN began as Alexander Graham Bell’s local exchanges and grew into an interconnected mesh of national telcos. Its circuit-switched logic still anchors phone numbering, call routing and lawful intercept rules worldwide.
For outsourcing buyers, the PSTN still matters. Regulators in many markets require contact centre operators to terminate certain emergency numbers (000, 911, 999, 112) over circuit-switched lines rather than the public internet.
Key takeaways
- PSTN reserves a dedicated circuit per call, so quality holds steady even when the public internet is congested.
- Carriers in the US, UK and EU are retiring copper lines and pushing voice onto IP networks.
- Philippine and Indian contact centres still buy PSTN trunks for toll-free inbound numbers and emergency compliance.
- One voice channel carries 64 kbps, which is why dial-up modems topped out at 56 kbps.
- Copper lines draw power from the exchange, so a corded handset works through a local blackout.
How it works
The PSTN builds a dedicated electrical or optical path between two phones for the length of a call. A hierarchy of local exchanges, tandem switches and international gateways holds that path open until someone hangs up.
Signalling System 7 (SS7) tells those switches where to send each call, while the E.164 numbering plan keeps every line uniquely addressable across borders. Those two standards are why a Manila trunk can ring a Kansas landline.
- You lift the handset and the local exchange detects the off-hook signal.
- You dial, and 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 network can’t multiplex: one call holds one channel for its full duration.
Most business PSTN access still arrives as copper twisted pair or an Integrated Services Digital Network (ISDN) circuit, which bonds two 64 kbps channels into one 128 kbps link for voice and data.
| 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 |
| Interconnect | Hands calls to mobile and IP networks | Media gateway, softswitch, session border controller |
The International Telecommunication Union, the United Nations agency for telecom standards, sets the numbering, signalling and quality rules that let national networks interoperate.
National regulators translate those rules into licence conditions — the FCC in the US, Ofcom in the UK, and the National Telecommunications Commission in the Philippines.
Copper retirement is a licensing question as much as an engineering one. In the United States the FCC requires carriers to notify customers before withdrawing a copper facility, which is why timetables slip from state to state.
Examples
Every big carrier sits somewhere on the same journey: keep the copper running for regulated traffic, move everything else to IP. These four cases show how differently national timetables have played out since 2018.
BT Openreach (UK). BT, the UK’s incumbent fixed-line carrier, set a December 2027 deadline to retire the legacy PSTN and move every landline to Digital Voice over IP. Ofcom confirmed that timetable after an earlier pause.
AT&T (United States). AT&T, the largest US wireline carrier, has asked federal regulators to retire copper across most of its 21-state footprint, replacing it with fibre and fixed wireless. Waivers have been granted market by market rather than nationally.
Globe Telecom and PLDT (Philippines). Both Manila carriers still run a substantial PSTN footprint for business process outsourcing clients. Toll-free inbound numbers terminated in Manila and Cebu stay anchored to circuit-switched trunks.
Deutsche Telekom (Germany). Germany’s incumbent operator finished its all-IP migration in 2018, years ahead of most European peers. Voice now runs over an IP Multimedia Subsystem (IMS) core — though emergency calls still touch PSTN gateways.
The buyer’s question is the same in every market: which trunks carry your emergency and toll-free traffic, and when does the carrier plan to cut the copper?
A switch-off dated 2027 can strand a five-year voice contract signed today, so migration terms belong in the contract, not in a later change request.
Related terms
PSTN sits inside a cluster of voice and outsourcing terms, and most buyer confusion comes from mixing them up. These seven neighbours cover the protocols, the hardware and the commercial layer that surround a circuit-switched line.
- Voice over Internet Protocol (VoIP): the IP-based successor that carries voice as data packets.
- Contact Centre: the operation that routes inbound and outbound calls across PSTN and IP trunks.
- Business Process Outsourcing: the wider discipline whose voice providers buy PSTN and SIP capacity.
- Interactive Voice Response: the menu layer sitting between a PSTN trunk and a live agent.
- Toll-free Number: the inbound number type whose routing still depends on PSTN translation databases.
- Session Initiation Protocol: the signalling standard doing for IP voice what SS7 did for PSTN.
- Automatic Call Distributor: the queueing hardware historically wired straight into PSTN trunks.
FAQ
Is the PSTN being shut down?
Yes, in stages. Germany finished in 2018, the UK is targeting December 2027, and the US is running a rolling, state-by-state copper retirement under FCC oversight. Many developing markets will keep circuit-switched lines running well 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 splits speech into packets and ships them across an IP network. PSTN is the more predictable of the two; VoIP is cheaper and far easier to scale.
Why do contact centres still use PSTN?
Regulators in most countries require emergency numbers, and some toll-free ranges, to terminate on circuit-switched lines. Plenty of enterprise voice contracts also pre-date the IP migration, so providers keep mixed PSTN and SIP trunks until renewal.
How fast is a PSTN line?
A single voice channel carries 64 kbps of digitised audio, which is why dial-up modems stalled at 56 kbps. An ISDN line bonds two channels for 128 kbps. None of those figures compete with modern broadband.
Does PSTN work during a power cut?
Copper PSTN draws power from the exchange, so a corded handset still rings during a local blackout, while fibre replacements need battery backup.
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