FXS is the jack on the wall that provides the dial tone you hook a plain old telephone (POTS) too.FXO is the jack on the phone itself that receives the dial tone.Essentially the FXO and FXS ports were connected in the POTS world.

The terms FXS and FXO are used most these days with regards to IP telephony and this is where most people get confused.

To hook an IP phone system to the POTS dial tone you will need a FXO Gateway.

To hook a POTS phone/phone system to a VOIP line you will need a FXS Gateway.

To hook a POTS phone to an IP phone system you will need a FXS Adapter.