What do the Terms FXS and FXO mean?

FXS (Foreign Exchange Subscriber) is the port that delivers the analog line to the subscriber. It provides dial tone, battery current, and ringing voltage. You connect your analog devices, like a phone or fax machine, to the FXS port.

FXO (Foreign Exchange Office) is the port on the device—such as a phone or fax—that receives the line. It connects to the Public Switched Telephone Network (PSTN). The FXO port sends on-hook and off-hook signals to control calls.

FXS and FXO ports always work in pairs. The FXS port sends the service. The FXO port receives it. If you plug an analog phone into the wall jack, you’re connecting the FXO side (the phone) to the FXS side (the jack).

When you make a call, the FXO port on the phone goes off-hook. The FXS port detects this and sends a dial tone. Once you dial the number, the FXS port processes the digits and connects the call.

On an incoming call, the FXS port rings the phone. The FXO port on the phone detects the ringing and signals when the phone is answered. This process allows two analog devices to communicate using a standard telephone line.

A phone call received via an analog line is initiated like this: Pick up the phone of the FXO device. The FXS port on the corresponding sides recognizes that the call should be established and expects the number, encoded as DTMF signals.

An incoming call works like this: The FXS port provides the necessary electricity. The phone starts ringing. By picking up the phone, the call is established.

You’ll come across the terms FXS and FXO when choosing equipment to connect analog phones to a VoIP phone system, or to link traditional PBXs with a VoIP Phone System provider or with each other over the Internet.

To connect an analog fax machine to your phone system, you need at least one FXS port. To use analog telephone lines with your system, you need one FXO port for each line.

To connect analog phone lines to an IP phone system , use an FXO gateway. It links the FXS port to the FXO port on the gateway, converting the analog phone line into a VoIP call. Several FXO gateways are available. You can view the list of gateways supported by 3CX.

An FXS gateway is used to connect one or more lines of a traditional PBX to a VoIP phone system or provider. Alternatively, you can use it to connect analog phones to it and re-use your analog phones with a VoIP phone system. You need an FXS gateway because you want to connect the FXO ports (which normally are connected to the telephone company) to the Internet or a VoIP system.

An FXS adapter is used to connect an analog phone or fax machine to a VoIP phone system or to a VoIP Provider. You need this because you need to connect the FXO port of the phone / fax machine to the adapter.

FXS / FXO gateways are widely available. 3CX automatically configures FXS / FXO gateways to allow you to easily continue using your existing PSTN lines and/or analog phones. You can get your free licence here.

More information about FXS / FXO and VoIP, in general, can be found in our Basic Certification.

If you are interested to know in more technical detail how an FXS / FXO port interoperates, here is the exact sequence:

When you wish to place a call:

Inbound call:

Ending the call – normally the FXS port relies on either of the connected FXO devices to end the call.

Note: The analog phone line passes approximately 50 volts DC power to the FXS port. That’s why you get a faint “shock” when you touch a connected phone line. This allows a call to be made in the event of a power cut.