PSTN, or Public Switched Telephone Network, is the traditional circuit-switched phone system used for landlines and many enterprise voice calls. In modern environments it often connects with VoIP, SIP trunks, and contact-center platforms through carrier handoffs and gateways.
In cyber security, PSTN matters because it is part of the attack surface and the trust boundary for voice services. Weak number management, routing misconfigurations, and poor carrier controls can enable call interception, toll fraud, spoofed caller identity, or service disruption. Defenders monitor PSTN-connected systems to verify call routing, protect transfer paths, and preserve availability and call quality. When voice automation or incident response relies on phone channels, a clean PSTN path is essential for reliable authentication, customer contact, and failover.