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FXS(Foreign eXchange Station) The interface provided by the telephone company to its customers, which includes dial tone, power and ring voltage. The telephone jack on the wall is an FXS interface. The plug on a telephone is a Foreign Exchange Office (FXO) interface, which provides on-hook/off-hook loop closure to the telephone company. Thus, an FXO phone plugs into an FXS jack.
A PBX's FXO interface is connected to the FXS interface, and all phones (FXO) connect to the FXS interface on the PBX.