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FXO
In telecommunications, a Foreign Exchange Office, or FXO, is a telephone signaling interface that receives POTS, or "plain old telephone service". It generates the on-hook and off-hook indicators used to signal a loop closure at the FXS's end of the circuit. Analog telephone handsets, fax machines and (analogue) modems are FXO devices, though the term is rarely used except in connection with Foreign exchange service (FX).
FXO interfaces are also available for computers and networking equipment, to allow these to interact directly with POTS systems. These are commonly found in devices acting as gateways between Voice over Internet Protocol (VoIP) systems and the public switched telephone network (PSTN).
FXS
In telephony, a Foreign eXchange Station, or FXS, is a telephone interface which supplies battery power, provides dialtone, and generates ringing voltage. A device that connects to such an interface contains an Foreign exchange office (FXO) interface and could be a standard analog telephone or a private branch exchange (PBX) to receive telephone service.Any telephone exchange is an example of an FXS, as is the telephone jack on the wall, though the term is rarely applied except in connection with foreign exchange service.An FXS interface utilizes an FXO protocol to detect when the terminating device (telephone) goes on-hook or off-hook, and can send and receive voice signals.