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If you want to connect your phone line to your VoIP equipment (e.g. Asterisk server) so that it can make and answer calls on your telephone line, you need to add a FXO interface to your VoIP equipment. Similarly, if you want to integrate your VoIP equipment with your existing telephone system which has a spare extension line, you also need a FXO interface.

On the other hand, if you want to connect an ordinary telephone set to your VoIP equipment, you need a FXS interface.

An FXO gateway converts the regular, public switched telephone network (PSTN) into one of VoIP variants.
An FXS gateway does exactly the opposite - it takes VoIP signal and transforms it back to regular "phone line" signal.

When a customer receives phone service from a central office other than the one that would normally serve them, the line between the customer and the "Foreign" office is called a "Foreign Exchange" line.

FXO (Foreign eXchange Office) is the office end of the line, and FXS (Foreign eXchange Station) is the station end.

FXO and FXS are also used to refer to the type of interface on phone equipment.

An FXS interface provides power (battery) to FXO equipment and generates ring signals
(e.g. The phone jack on the wall)

An FXO interface receives power (battery) and receives ring signals
(e.g. The phone line port on a telephone set)

Always check the sense of the device whether it is labeled FXS or FXO, as some vendors get it backwards and your device will provide battery when you expect it to behave as a station and receive battery. Be careful!

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