private branch exchange

from The Free On-line Dictionary of Computing (8 July 2008)

Private Branch Exchange
PBX

   <communications> (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)