PSTN (public switched telephony network)
The PSTN is the aggregate of the world’s circuit-switched telephone networks. The technical operation of the PSTN adheres to the standards created by the ITU-T. These standards allow different networks in different countries to interconnect seamlessly. The E.163 and E.164 standards provide a single global address space for telephone numbers. The combination of the interconnected networks and the single numbering plan make it possible for any phone in the world to dial any other phone.