foreign exchange subscriber (FXS)
Foreign Exchange Subscriber (FXS) or Foreign Exchange System (FXS) is a common interface in analog telephone systems.
Plain Old Telephony Service( POTS) is the telephone service provided by local operating companies to their telephone subscribers. Telephony is provided as a separate service of the local telephone companies from their local exchange(OVSt) to the subscriber in his building over copper twisted pairs.
The Foreign Exchange Subscriber Interface is the telecommunications connection unit through which the telephone service is connected to the subscriber equipment, the telephone, fax machine or modem. It is an FXS interface for the subscriber through which the dial tone and voltages are routed.