n telecommunications, Foreign Exchange Office, or FXO, designates a telephone signaling interface that receives POTS, or "plain old telephone service". It generates the off-hook and on-hook indications (loop closure/non-closure) at the FxS's end of a telephone 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).

[edit] In a nutshell

An FXO device is any device that, from the point of view of a telephone exchange, appears to be a regular telephone. As such, it should be able to accept ringing signals, go on-hook and off-hook, and send and receive voice frequency signals. It may use loop start or ground start signaling. FXO channel units were invented and named in the middle 20th century for service at the "Office" end of an FX line via carrier system.