A telephone exchange is telecommunications equipment that connects and switches phone lines between callers and receivers. It serves as the heart of the telephone network, analyzing dialed numbers to select optimal routing and establish calls. It is the central component of the PSTN (Public Switched Telephone Network).
The history of telephone exchanges is the history of telephony itself. The world's first telephone exchange opened in Connecticut, USA, in 1878, where human operators manually connected lines using plugs and jacks. In Japan, telephone exchange service began between Tokyo and Yokohama in 1890. Japan's first automatic exchange was installed in 1926, enabling direct dialing. Subsequent evolution through step-by-step exchanges, crossbar exchanges, and electronic exchanges (such as NTT's D70) dramatically increased processing speed and line capacity.
Today's telephone exchanges are predominantly softswitches (software-based exchanges). While traditional hardware exchanges physically connected lines using dedicated circuits, softswitches manage call control through software running on general-purpose servers. They use the SIP protocol to manage call setup and teardown, with voice data transmitted as IP packets over trunk lines.
NTT completed the migration of its landline network to IP in 2024. This replaced telephone exchanges nationwide with softswitches, transitioning from circuit-switching (dedicating a line for each call) to packet-switching (dividing data into small packets transmitted over shared lines). Enterprise telephone exchanges - PBX systems - are also accelerating their migration to cloud PBX. See Phone Number Structure for the overall telephone network and History of Phone Number Digit Lengths for the evolution of the numbering system.