Centrex service

In telecommunication, a Centrex service (CTX) is a service offered by Bell Operating Companies that provides functions and features comparable to those provided by a PBX or a PABX. Note: "Centrex C.O." indicates that all equipment except the attendant's position and station equipment is located in the central office. "Centrex C.U." indicates that all equipment, including the dial switching equipment[?], is located on the customer's premises.

Source: from Federal Standard 1037C