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