Foreign exchange office

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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).

[edit] In a nutshell

An FXO is any device that, from the point of view of a telephone exchange, seems 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. As with other lines, it may be loop start or ground start. 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