What is PSTN? The public phone network, and how VoIP calls get into it

What is the PSTN?

The PSTN is the public phone system you reach by dialling a number: the fixed and mobile networks of every country, joined by one numbering plan ^[4]. The ATIS telecom glossary defines it as a network usually accessed by telephones, office phone systems and data connections, and says the worldwide network is based on ITU-T E.164 ^[1][1]. Anything with a dialable number sits on the PSTN or behind a gateway to it.

Switched means the network completes a circuit between caller and callee, which takes signalling such as dial pulses or tones ^[1]. Between digital exchanges, the ITU-T standard for that signalling is Signalling System No. 7 (SS7) ^[9]. The classic voice format is G.711: 8,000 samples a second at eight bits each, or 64 kbit/s ^[10].

In the UK the word is narrower: the government uses PSTN for the analogue landline network now being retired ^[8]. In standards and US rules it means the whole public phone system that VoIP services connect to ^[11].

PSTN vs VoIP: what is the difference?

The PSTN sets up a circuit and routes calls by phone number; VoIP sends voice as internet packets and routes by account or address. The two meet at gateways. US rules call a VoIP service interconnected when its users can receive calls from the PSTN and make calls to it ^[11]. App-only services work differently: WhatsApp Web, for example, calls your contacts on WhatsApp ^[12].

Digital home landlines run on VoIP or all-IP technology too ^[8]; I weigh them in a VoIP line versus a copper landline. Wi-Fi Calling is a mobile-carrier feature, not a browser one ^[15], as I explain in Wi-Fi Calling compared with browser calls.

Reaching the PSTN does not mean reaching 911, 112 or 999: TwinPhone cannot dial emergency numbers ^[16], neither can free Google Voice ^[17], and WhatsApp's terms exclude emergency services ^[18]. What to use instead is in my guide to emergency calls from a computer.

How does a VoIP call get onto the PSTN?

Through a gateway. The provider receives your call over the internet, usually passes it over a SIP trunk to a carrier, and the carrier that holds the number you dialled rings the phone. Twilio describes its SIP trunks as connecting IP-based phone systems to the PSTN for making and receiving calls ^[2]. The browser half is in my step-by-step on browser calls; here is the phone-network half.

1. SIP sets up the call to the gateway. It establishes, modifies and ends internet calls, and the SIP standard itself describes calls connected to a PSTN gateway ^[13].
2. The number goes out in E.164 form. Twilio's trunking documentation calls the leading plus sign a must ^[2].
3. The audio meets the phone network's codec. WebRTC browsers must support G.711 as well as Opus ^[14], and G.711 is the classic 64 kbit/s telephone format ^[10].
4. The country code picks the destination. In E.164 it selects the country where the number is registered ^[4].
5. The terminating carrier rings the phone. It switches the call and delivers it to the line you dialled ^[19], and this is the leg that costs money.

TwinPhone, which sponsors this site, describes the same handover: it bridges the encrypted browser call to the public phone network, where the final carrier leg follows the recipient carrier's security model ^[20]. Sonetel's help describes an onward call across the regular telephone network ^[21].

The gateway also decides which number the other person sees: Twilio lets a trunk send only a number it issued or one the customer verified ^[2], while PopTox shows one shared number on every call ^[22]. I cover the options in which number appears when you call from a browser.

One number at each step

The number is only an example. International prefixes for other countries are in my guide to calling abroad online.

Who gets paid when an internet call ends on a phone?

The carrier that holds the number you call. Delivering a call on its network is called termination, and operators charge each other for it ^[3]. An app-to-app call never reaches a carrier, so there is no such charge. A call to a landline or mobile does, mobile termination usually costs more ^[3], and the per-minute price you pay covers it.

Rules differ by region. US rules define bill-and-keep arrangements, in which carriers exchanging traffic do not charge each other for termination ^[25]. In the EU, termination is capped at $0.002 a minute on mobile networks and $0.001 on fixed ones, and the caps do not cover calls from non-EU numbers ^[3].

I read these rate cards and placed no calls. Yolla lists from prices ^[5] and TwinPhone its lowest rate for each destination ^[6], so some numbers cost more. Prices are converted to your currency; the original is in the tooltip.

Free web dialers ration that paid leg: PopTox allows five free calls a day to the same number ^[29], and Call2Friends one or two a day per IP address ^[30], two minutes each to most US area codes ^[31]. TwinPhone has no free calls or trial credit ^[16]; its UK landline price matches Yolla's and its UK mobile price is higher, and I compare free limits in my look at short free-call offers.

WhatsApp Web is the app-to-app case: calls to WhatsApp contacts are free, even in another country ^[12]. I found no option in its help pages to ring an ordinary number; more in whether WhatsApp can reach a landline.

Is the PSTN being switched off?

The analogue part is; the network of numbers is not. In the UK, the telecoms industry announced in November 2017 that it would retire analogue networks such as the PSTN, and most customers are expected to have switched to digital lines by the end of January 2027 ^[8]. Landline service itself continues ^[8], so a browser call to a landline still ends with a carrier ringing that line.

UK landlines move to VoIP, Digital Voice or all-IP service ^[8], and a landline-only option remains for customers who do not want an internet connection ^[8]. In the US, an FCC order published in April 2026 cut regulatory barriers to moving from legacy networks to IP-based infrastructure ^[32].

The real difference is power. An analogue line carries enough current from the exchange to run some basic corded phones in a power cut; a digital landline cannot, so it needs mains or backup power ^[8]. A browser call has the same limit: no power for the router, no call, so a charged mobile is the safer backup for emergencies.

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