Key telephone systems are (usually small) multi-line private telephone network that allows each key telephone station to select one of several telephone lines, place a line on hold, and call via an intercom circuit between key telephones. Key systems contain a central key service unit (KSU) that coordinates status lights and lines to key telephones ( Key Sets ). Early KTS system technology was based on electromechanical relay hardware. They required all the outside telephone lines to be connected to all of the key telephone sets in the installation. In addition, two additional pairs of wire were used in conjunction with each telephone line, one pair for the A/A1 connection indicating if that line is off hook at that particular key telephone set, and another pair to operate a small light to indicate the status of that line. Consequently, each key telephone set was connected to the central KSU via a thick cable containing 50 wires (25 pairs). Newer KTS systems typically use only 4 wires to connect the electronic KSU to each electronic key telephone set, and are often called skinny wire key systems. Modern electronic key systems are small microprocessor controlled switching systems and have some of the same advanced call processing features such as call hold, busy status, multi-line conference, abbreviated dialing, and station-to-station intercom that are available in a larger PBX.

See: Key Telephone System

A telephony service that allows a telephone user to automatically call back the phone...