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

FXO
       Foreign eXchange Office (FXS)


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

Foreign eXchange Office
FXO

    (FXO) An analog telephone plug on a handset
   that receives POTS service from the telephone exchange
   ("central office") via a Foreign eXchange Subscriber socket
   and provides on-hook/off-hook indication to the exchange.

   (2008-01-17)