The Public Switched Telephone Network (PSTN) is the worldwide, circuit-switched system that connects phone calls across local exchanges, long-distance carriers, and international gateways. If you’re asking what is PSTN, think of it as the legacy backbone that provides dial tone, phone numbers, and reliable call completion almost anywhere on earth.
Unlike VoIP, which sends voice as IP packets, PSTN establishes a dedicated path for the duration of the call. Mobile and VoIP calls often still interconnect with the PSTN to reach traditional numbers, emergency services, and regions without all-IP coverage. We often see enterprises keep limited PSTN access for life-safety lines, fax/modems, and failover while shifting most calling to SIP/UC platforms.
Key characteristics:
- Global numbering (E.164): Standardized phone numbers and country codes.
- High reliability: Central-office power and carrier-grade uptime.
- Emergency calling: Location-aware routing to local responders.
- Interoperability: Seamless reach to legacy and modern voice systems.
Our take? The PSTN remains essential for reach, reliability, and compliance—even as most enterprise voice migrates to IP.
Modernizing enterprise calling or enabling Teams as your softphone? Explore our Microsoft Teams PSTN Connectivity Guide to compare Direct Routing options, number management, E911, and failover patterns that blend cloud agility with PSTN reliability.
