FXS port Foreign exchange subscriber port.
Also known as foreign exchange subscriber interface:
(1) as an RJ-11 wall jack,
(2) as an RJ-11 port on a PBX to which a telephone, fax machine, or modem can be connected via an RJ-11 cable, and
(3) as an RJ-11 port on an analog telephone adapter to which a telephone or fax machine can be connected via an RJ-11 cable for connectivity to a VoIP system or to a VoIP provider