What is the PSTN?

PSTN stands for Public Switched Telephone Network, the original global phone system built from physical circuits and exchanges. Almost every call you have ever made has touched it somewhere.

The original phone network

PSTN stands for Public Switched Telephone Network. It is the traditional, globally interconnected system of copper wires, fibre trunks, and switching exchanges that has carried voice calls since the late 1800s, long before the internet existed. When people say "landline," this is the network underneath it.

How a PSTN call actually finds its way

Every PSTN phone connects to a local exchange. When you dial a number, that exchange looks at the digits and decides where to send the call: to another phone on the same exchange, or up the chain to a regional or long-distance switch that knows how to reach the destination exchange. This chain of switches is what reserves a dedicated circuit for your call from end to end.

The signaling that coordinates all of this between exchanges is a decades-old system called SS7, which is also the reason caller ID can be so easily faked, since SS7 was built assuming only trusted phone companies would ever touch it.

Why it is disappearing

Maintaining physical circuit-switched infrastructure is expensive, and carriers worldwide are steadily shutting down PSTN exchanges in favour of routing everything over IP networks instead, even when the call still ends up sounding like an ordinary phone call to you.