The traditional phone lines, the analogue POTS service, used to connect old telephone to the telephone exchange. This type of analogue line is still in use by many types of devices. Sometimes terms FXO and FXS are used. Here is the definition and explanation of what they are:
FXS - Foreign Exchange Station
FXO - Foreign Exchange Office
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 phone system to receive voltage from the outside lines.
Connect the phones to FXS ports on your system or gateway. When the FXO module in your system receives the voltage, it will then generate voltage using the FXS module and send it to your analog phone.