What is the PSTN?
The Public Switched Telephone Network (PSTN) is the worldwide web of fixed and mobile telephone operators that lets any phone number reach any other. The name describes how it was built: calls were switched between interconnected exchanges, first by human operators at switchboards and later by automatic switches, and every line was reachable through a public numbering plan — today the E.164 format of a country code followed by up to 15 digits in total.
Little of today's PSTN looks like the copper network it is named after. Exchanges went digital in the 1970s and 1980s, call set-up moved onto a separate signalling network (SS7), mobile networks joined in the 1990s, and operators now carry most voice traffic as IP inside their cores. What survives is the part that matters to a business: the shared numbering, interconnection between operators and the promise that a call reaches any handset — including feature phones with no data plan.
For CPaaS and messaging teams, the PSTN is where voice traffic ends up. Voice OTP calls, flash call verification, Interactive Voice Response (IVR) lines, missed-call numbers and outbound sales calls all terminate on it. Internet calls get there through a gateway or SIP trunk that translates Session Initiation Protocol (SIP) signalling and RTP audio into what the operator expects; the SIP page covers that protocol, while this page covers the network it connects to. WhatsApp calls, including those made through the WhatsApp Business Calling API, are a separate path: they run between apps over the internet and are not PSTN calls unless a business bridges them into its own phone system.
Key takeaways
- The PSTN is the interconnected network of fixed and mobile operators that lets any phone number call any other.
- Its copper and analogue parts are being retired, but its numbering, interconnection and reach to every handset still carry everyday calls.
- Internet calls reach the PSTN through gateways and SIP trunks, and that hand-off is where per-minute costs and caller-ID rules apply.
- Voice OTPs, IVR lines and flash calls all terminate on the PSTN; WhatsApp calls do not, because they run between apps over the internet.
- The UK plans to switch off its copper PSTN by 31 January 2027, moving landlines to digital voice while numbers keep working.
Figure 1
150 years of the phone network, in nine stops
Jan 2027Coming upEvent 9 of 9
UK copper switch-off
The UK's legacy copper PSTN is scheduled to be switched off by 31 January 2027, with landlines moved to digital voice over broadband. Numbers keep working.
Event 9 of 9: Jan 2027 — UK copper switch-off (coming up). The UK's legacy copper PSTN is scheduled to be switched off by 31 January 2027, with landlines moved to digital voice over broadband. Numbers keep working.
Example: A voice OTP fallback for a lender in Lucknow
A lending app in Lucknow verifies borrowers by SMS OTP, but some applicants in low-coverage villages never receive the code. The team adds a "Call me with the code" button. When a borrower taps it, the backend asks its voice provider to place a call; the provider sends a SIP INVITE across its trunk, the call enters the PSTN through the carrier, and the borrower's basic feature phone rings. Text-to-speech reads the six digits slowly, twice, because the last leg is narrowband audio. The team budgets the OTP's validity for ring time plus playback, logs the SIP response code of every failed call (busy, no answer, invalid number), and compares the per-call termination cost with the SMS it replaces.
How does the PSTN work?
A number identifies the line
Each subscriber has an E.164 number. When a call starts, the network works out which operator serves that number today — number portability means the prefix alone no longer tells you.
Signalling sets up the call
Exchanges agree a route and reserve capacity before any audio flows. Historically that was SS7 (ISUP) messages between switches; between modern operators it is increasingly SIP over IP interconnects.
A path carries the voice
The classic PSTN voice channel is 64 kbit/s G.711 audio sampled at 8 kHz. Cores now move it as packets, but any narrowband leg caps quality, so high-definition audio rarely survives end to end.
Gateways bridge VoIP to the PSTN
A VoIP system, a CPaaS platform or a contact centre hands the call to a carrier through a media gateway or SIP trunk. This is where termination rates, caller-ID presentation rules and number-format checks apply.
The far end rings, and billing starts on answer
Calls are usually charged from the moment they are answered, in billing increments set by the carrier. That is why flash calls, which are dropped before answer, became a cheap verification method — and why operators in several countries now push back on them.
Why it matters
Why does the PSTN matter?
Every voice feature a business builds — an OTP call for users whose SMS never arrived, a missed-call opt-in line, an IVR, a cloud telephony desk calling leads — depends on reaching the PSTN, and the PSTN sets the rules. Numbers must be valid and correctly formatted, caller ID is controlled by operators and regulators, audio is squeezed to narrowband on many legs, and calls are billed per minute or per pulse by destination. Knowing where your call leaves the internet and enters the phone network explains most voice costs, delays and caller-ID problems.
Common mistakes with the PSTN
Assuming caller ID arrives as you sent it
The originating carrier decides what the called party sees, and many networks block or overwrite numbers you are not authorised to present. Present only numbers your provider owns or has verified for you, and test what actually shows on each major operator.
Ignoring call set-up time
A PSTN call takes several seconds to route and ring before anyone answers. A voice OTP that expires in 30 seconds can die before it is read out; size validity for set-up, ringing and playback.
Modelling cost by minutes alone
Carriers bill in increments (per minute, per 30 seconds, per pulse) with minimum durations, and rates differ sharply by destination and number type. Ask for the billing increment before comparing voice with SMS or WhatsApp.
Expecting HD audio on every call
Once a call crosses a narrowband leg, wideband audio is transcoded down to G.711. Write voice prompts for 8 kHz audio: short sentences, digits read one by one, key information repeated.
Treating WhatsApp calls as phone calls
WhatsApp calls need the app and a data connection and never ring a landline or a feature phone. For users without data, a PSTN call or SMS is the only voice-or-text route left.
Best practices for the PSTN
- Store every number in E.164 so the same record works for SMS, voice and WhatsApp.
- Read voice OTP digits one at a time and repeat them; narrowband audio swallows fast speech.
- Log the SIP response code and reason for every outbound call; it explains most failed PSTN calls.
- Check your carrier's billing increment and minimum duration before forecasting voice costs.
PSTN FAQs
- Is the PSTN still used today?
- Yes. Copper lines and analogue switches are being retired, but the PSTN's numbering, operator interconnection and reach to every handset still carry everyday voice calls. Even the UK's switch-off, scheduled for 31 January 2027, replaces copper landlines with digital voice rather than abolishing phone numbers.
- What is the difference between the PSTN and VoIP?
- The PSTN is the public operator network you reach by dialling a phone number; VoIP is a way of carrying voice as internet packets. Most VoIP systems still connect to the PSTN through gateways or SIP trunks so their users can call ordinary numbers.
- Do WhatsApp calls use the PSTN?
- No. WhatsApp calls, including calls through the Business Calling API, travel over the internet between the WhatsApp app and the business. A business can connect them to its own phone system over SIP, but the customer's side never touches the public phone network.
- What does PSTN termination mean?
- Termination is the last leg of a call, where it is handed to the operator that serves the called number. Termination rates vary by country, operator and number type, and they are usually the biggest part of the cost of a voice OTP or outbound call.