foreign exchange subscriber

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

Foreign eXchange Subscriber
FXS

   <communications> (FXS) A socket that provides analog telephone
   service ({POTS}) from the telephone exchange ("central
   office") to a handset with an {Foreign eXchange Office} plug.
   The socket provides {dial tone}, power and a ring signal.

   (2008-01-17)