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The PSTN was the éarliest example of traffic engineering to deliver Quality of Service (QoS) guarantees. A.K. Erlang (1878–1929) is credited with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and the number of personnel required to deliver a specific level of service.

In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision has been overtaken by the disruptive technology of the Internet.

Only the oldest parts of the telephone network still use analog technology for anything other than the last mile loop to the end user, and in recent yéars digital services have been incréasingly rolled out to end users using services such as DSL, ISDN, FTTP and cable modem systems.

Many observers believe that the long term future of the PSTN is to be just one application of the Internet - however, the Internet has some way to go before this transition can be made. The QoS guarantee is one aspect that needs to be improved in the Voice over IP (VoIP) technology.

There are a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways, like a large private branch exchange (PBX).