FXS - Foreign Exchange Subscriber interface (the plug on the wall) delivers POTS service from the local phone company's Central Office (CO) and must be connected to subscriber equipment (telephones, modems, and fax machines). In other words an FXS interface points to the subscriber. An FXS interface provides the following primary services to a subscriber device: Dial Tone, Battery Current ,Ring Voltage FXO - Foreign Exchange Office interface (the plug on the phone) receives POTS service, typically from a Central Office of the Public Switched Telephone Network (PSTN). In other words an FXO interface points to the Telco office. An FXO interface provides the following primary service to the Telco network device. FOR EXAMPLE: The telephones( 90201\90202) connect the FXS PORTS, the PBX extensions(26955556-90203\26955554-90204) connect the FXO PORTS.