Foreign Exchange Subscriber (FXS) and Foreign Exchange Office (FXO) are the two most common interfaces discussed in analogue telephony environments. The usual query is – What is the difference between FXO and FXS?


The comparison below will clear some of the queries on both interface types –



Foreign Exchange Subscriber (FXS) interface delivers POTS service from the phone company’s Central Office (CO) and connects 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

Foreign exchange Office interface receives POTS service from Central Office of the Public Switched Telephone Network (PSTN)  An FXO interface provides the following primary service to the Telco network device:

  • On-hook/off-hook indication (loop closure)

FXO and FXS interface go hand in hand to complete a connection. If we connect an FXS device to another FXS device, the connection will not work. Likewise, if FXO device is connected to another FXO it will not work. Hence, one cannot plug a standard analogue telephone (FXO) directly into a standard analogue telephone (FXO) and talk phone-to-phone. A telecommunications line from an FXO port must connect to an FXS port in order for the connection to work. Similarly, a line from an FXS port must connect to an FXO port in order for the connection to work.

Related- Commonly Used VOIP Terms and Concepts

Below video will also help enlighten understanding of FXS and FXO interfaces and their usage in telecom –

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