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
(e.g. The phone jack on the wall)
An FXO interface receives power (battery) and receives ring
(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!