The public switched telephone network (PSTN) is the umbrella network covering the various commercial and government-owned public circuit-switched telephone networks around the world. It includes mobile and fixed "landline" telephones. The service carried by the PSTN is often called Plain Old Telephone Service (POTS). Much of the Internet's traffic actually passes through much of the PSTN's long distance and local (in the case of dial-up internet access) infrastructure.

The PSTN uses a single global numbering system based on E.163 and E.164 standards. The numbering system is geo-politically defined, with number prefixes referring to political entities. Other technical standards created by the ITU-T allow the different networks around the world to interconnect and pass calls between them seamlessly. The interconnected networks in tandem with the single numbering system allow any telephone in the PSTN to dial any other telephone in the PSTN.