CENTRal EXchange. Telephone company service that renders obsolete the need of buying, maintaining, and upgrading a private branch exchange (PBX). Essentially, Centrex service consists of ordinary trunk lines with several features added by the phone company. These features include the common four: call conferencing, call forwarding, call waiting, and speed calling, as well as others such as call accounting, caller ID, call hold, call transfer, direct inward dialing, intercom, least cost routing, and so on. More economical than a PBX system, Centrex does require special telephone equipment for exploiting its features.