An FXS device initiates and sends ringing voltage. An FXO device receives the voltage sent by the FXS device.
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.