V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

FXS
       Foreign Exchange System / Subscriber (FXO)


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

Foreign eXchange Subscriber
FXS

    (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)