At the beginning, the primary advantage of PBX was to save the consumption of internal calls: to process the internal line conversion of the system to reduce telephone charges. When PBX gradually became popular, some new functions were added, such as intelligent line finding, user calling, and dialing extensions, which were not provided by telephone companies.

PBX, also known as SPC exchange, completes telephone exchange between enterprises and public telecommunication network, and combines telephone, fax, modem and other functions. General term ext (extension) refers to the extension connected to the main line. The PBX is to handle the calls between extensions and then connect with the Public Switched Telephone Network (PSTN) through the trunk line.