PSTN
The PSTN, or public switched telephone network, is the worldwide collection of public telephone networks that assigns dialable numbers and routes calls among fixed, mobile, and business phone systems.
How the PSTN carries a call
When someone dials a telephone number, the originating carrier interprets that number and selects a route toward the receiving carrier. Several networks may participate before the destination rings. Call-control signals establish, manage, and end the connection, while a separate media path carries the speakers' audio.
The word “switched” comes from the network's job of connecting an origin to a destination for the duration of a call. Much of that infrastructure now uses digital and internet-based transport internally, but a call can still be a PSTN call because it enters the public numbering and carrier-routing system. PSTN therefore describes a service and network boundary, not one particular cable or transmission technology.
Why it matters for AI phone calls
An AI phone agent needs a bridge to the PSTN if ordinary phone users should be able to call it, or if it should place calls to ordinary telephone numbers. That bridge is commonly provided through a carrier connection or a SIP trunk. The AI application handles the conversation, while the telephony layer handles ringing, answer status, call signaling, audio delivery, and termination.
This separation explains why a working browser voice experience does not automatically mean a phone deployment is ready. Public calling adds number assignment, carrier routing, caller identity, regional availability, and failure conditions such as busy signals or unreachable destinations. It can also expose audio to codec changes and network handoffs that are absent in a direct web session.
For buyers, the practical question is not merely whether an agent can speak. It is whether the full setup can reliably receive or place calls through the public telephone system, using the intended numbers and routing rules.