What are FXS & FXO?

FXS — Foreign Exchange Station

FXO — Foreign Exchange Office

An FXS device initiates and sends ringing voltage. FXS sends the voltage to an FXO device, which receives it.

The phone receiving the call is the last FXO device in the chain, and when it receives voltage from an FXS device, the phone will ring.

Connect the outside line to an FXO port on your Asterisk server to receive voltage from the outside lines.

Connect the phones to FXS ports on your Asterisk server. When the FXO module in your Asterisk Server receives the voltage, it will then generate voltage using the FXS module and send it to your analog phone.

Hint: Remember the “to”… FXS = FX(to)Station and FXO = FX(to)Office.
With “to” representing the direction taken by the signal.