FXO and FXS¶

FXO (foreign exchange office) and FXS (foreign exchange station) are the two different ends of a telephone line: a telephone line is always FXO ↔ FXS, neither FXO ↔ FXO nor FXS ↔ FXS. The difference between these is about electronic power and signaling, not the functionality of the device:

• FXS: generates power and generates the ringing signal. Example: wall plug
• FXO: consumes power and generates the on/off-hook signal. Example: phone

However, a private branch exchange (PBX) can also have FXO ports: when a building has a few leased analog lines to the outside world but it has digital infrastructure internally, it can accept incoming calls from outside and routes them to the internal digital phone network, and vice versa. In summary, any explanation that says "FXO are end-user phones, the network side is FXS" is wrong, because the network can also include FXO interfaces.

The terms FXO and FXS are used in traditional telephony infrastructure and VoIP. The original meaning of both abbreviations do not make much sense in modern usage, because an analog telephone adapter (ATA) with the FXS interface is usually on the same desk as the phone itself, which is not very foreign.

Some Cisco equipment also has so called FXO failover bypass ports in case of a power failure, then the FXO ports are switched directly through, connecting some phone(s) to the outside lines.