The Collaborative International Dictionary of English v.0.48:

centrex \centrex\ n.
   a contracted form of CENTRal EXchange.
   [WordNet 1.5] Centric

WordNet (r) 3.0 (2006):

centrex
    n 1: (CENTRal EXchange) a kind of telephone exchange

The Free On-line Dictionary of Computing (30 December 2018):

Central office exchange service
Centrex

    (Centrex) A PBX service providing
   switching at the central office instead of at the company
   premises.  Typically, the telephone company owns and manages
   all the communications equipment necessary to implement the
   PBX and then sells various services to the company.

   (1999-10-27)