Public switched telephone network (PSTN)
The PSTN is the umbrella term for the public telephone network, run by carriers, that connects landlines and mobile phones to one another.
The public switched telephone network (PSTN) is the combined set of carrier-run networks that connects the world's landlines and mobile phones. It is the shared infrastructure that lets you dial any number and reach another. The name comes from classic copper lines, but much of today's PSTN runs on digital and IP-based systems behind the scenes.
How it works
When someone dials a number, their carrier routes the call to the carrier and line that number belongs to. An internet-based phone system or call platform does not connect to the PSTN directly; a carrier sits in between. The carrier delivers calls from the PSTN to the platform over a SIP trunk and carries the platform's outgoing calls back out. At that handover point, the audio is converted from traditional telephony into VoIP packets.
Why it matters
Whatever phone your customers use, their call reaches you over the PSTN. That is why an AI voice agent or cloud call center needs a real number connected to the PSTN to be useful. The number type, the carrier and the connection method are all decided at this point; we compare the options in our guide to connecting your business number to an AI assistant.
In Speakline
Callers dial your number from an ordinary landline or mobile phone; the call reaches Speakline through your carrier's SIP connection and goes to the voice agent or your team according to your call flow. You can port your existing number, or keep it with your carrier and connect the line (BYOC).