Definition
The PSTN, or public switched telephone network, is the worldwide set of interconnected carrier networks that route calls between telephone numbers. Any phone with a number can reach any other through it, whichever carrier serves each side.
How the PSTN works
The PSTN is not one network. It is thousands of carrier networks that agree to connect to each other and to route by phone number. Numbers follow the plan in ITU-T E.164: a country code followed by a national number, at most 15 digits in all, usually written with a leading plus sign.
When someone dials, their carrier reads the number’s prefix and finds the network that owns it. Signaling between carriers has historically used a system called SS7, and the voice itself was carried as a 64 kbit/s circuit. The name “switched” comes from that circuit design. Today much of the path between carriers runs over IP, and many calls cross between old and new equipment on the way. The word PSTN has stayed as shorthand for “the phone network that has numbers”, as opposed to calls inside an app.
The edge is where a voice agent connects. Your agent does not appear on the PSTN directly. A carrier gives it numbers and passes calls over a SIP trunk, turning the call into SIP signaling and RTP audio.
Why it matters for voice agents
Three things that voice agents run into come from the PSTN rather than from the agent.
Audio is narrow. A call that crosses the PSTN is usually limited to telephony audio, so recognition has less to work with than in a browser.
Identity is thin. The number a caller shows is whatever the originating carrier passes along, which is why caller ID cannot be trusted as proof of who is calling.
Control is shared. Delays, dropped calls and number reputation depend on carriers you do not run. When something fails, a call that works from one carrier and not another points to the network, not the agent.
Example
An illustrative inbound call: a caller on a mobile network dials +1 415 555 0123. Their carrier routes by the country code and area code to the carrier that owns the number. That carrier sends the call to your trunk as a SIP INVITE, the agent answers, and audio flows back across the same carriers as 8 kHz speech.