Source : Free On-Line Dictionary of Computing

Private Branch Exchange
     
         (PBX) A telephone exchange local to a
        particular organisation who use, rather than provide,
        telephone services.  The earliest PBXs were manual ({Private
        Manual Branch EXchange}, PMBX) but are now more likely to be
        automatic ({Private Automatic Branch eXchange}).
     
        (1997-06-25)