PSTN
The PSTN (public switched telephone network) is the worldwide, interconnected network of public telephone systems (landline, mobile and the carriers between them) that a phone number is reachable on.
Also called: public switched telephone network, landline network, the phone network
When someone dials a number, the PSTN is what routes the call from their carrier to the one serving that number, whatever technology sits at each end. It began as copper lines and circuit switches and is now largely digital and carried over IP, but it remains the common addressing and routing fabric that makes any number callable from any other.
Internet-based voice (VoIP, WebRTC) reaches ordinary phones through PSTN gateways, so a service that "has a phone number" is one connected to the PSTN. Number porting, caller ID and emergency calling are all PSTN concepts.