The Public Switched Telephone Network (PSTN) is the global network of traditional circuit-switched telephone systems that connects landline and mobile voice calls worldwide. It is the legacy infrastructure reached by ordinary phone numbers, bridged to modern software through SIP trunks and voice APIs. Calls to or from a normal phone number ultimately traverse the PSTN.
Voice AI platforms integrate with the PSTN so AI agents can place and receive real phone calls to customers on any regular number — not just app-to-app audio. PSTN connectivity (usually via SIP or a telephony/voice API) is what lets an AI voice agent actually call a phone. Considerations include per-minute costs, number provisioning, global coverage, and call quality and latency.