What are FXS and FXO ports? What are they used for?

FXS and FXO are the names of the ports used by analog phone lines (also known as POTS – Plain Old Telephone Service) or phones. The two ports are always paired together in order to make a call.

FXS, Foreign Exchange Subscriber ,is a port that conveys the internal connection to office devices like analog fax machines or analog phones. It is a wall jack or FXS interface that delivers dial tone, ringing voltage, and battery power to FXO devices.

FXO, Foreign Exchange Office ,is the port that receives the analog line. It is the plug on the phone or fax machine or the plug(s) on your analog phone system. It delivers an on-hook / off-hook indication (loop closure). Since the FXO port is attached to a device, such as a fax or a phone, the device is often called the “FXO device.” This port establishes the connection to the analog line (FXS).

How Do FXS and FXO Ports Work?

In order to establish a call, the FXS and FXO ports must be connected. This applies to both outbound and inbound calls. When you pick up an analog phone (an FXO device) that is connected to an FXS port, you dial a number and connect to the other end using the PSTN service. The number you dial on your office phone is encoded as DTMF or Dual-Tone Multi-Frequency digits and passed to an FXS port.

In case of incoming calls, an FXS interface will transmit the ring voltage to the attached FXO device, and it will start ringing. Once the phone is picked up, the line is established.

FXO and FXS ports are used when a company wants to introduce an additional network. Such network systems can be Voice over IP or Private Branch Exchange (PBX). The connection procedure is slightly more complex and requires new lines attached to another network provider.

FXS and FXO Ports With VoIP Phone System

FXS Gateway

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 to the Internet or a VoIP system.

FXO Gateway

To connect analog phone lines to an IP phone system you need an FXO gateway. This allows you to connect the FXS port to the FXO port of the gateway, which then translates the analog phone line to a VoIP call.

When connected to a headquarter IP PBX system, FXO gateways can use the local landlines of your remote sites to avoid long distance rates. Besides, in the event of a network crash and SIP trunking to IP-PBX fails, FXO Gateway can offer a failover or fallback solution by connecting your PBX to an outside telephone line.

What are the FXS / FXO procedures?

When you wish to place a call:

  • You pick up the phone (the FXO device). The FXS port detects that you have gone off-hook.
  • You dial the phone number, which is passed as Dual-Tone Multi-Frequency (DTMF) digits to the FXS port.

Inbound call:

  • The FXS port receives a call, and then sends a ring voltage to the attached FXO device.
  • The phone rings.
  • As soon as you pick up the phone you can answer the call.

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


Foreign Exchange Subscriber and Foreign Exchange Office ports are connected in order to make or receive a call. An FXS port is a plug on the wall, and an FXO port is a plug on your office phone or a fax machine. When making a call, an FXO device sends a signal to an FXS port. And to answer a call, an FXS port sends a ring voltage to an FXO port to a device. However, the process is different when adding another network, like a VoIP system. An FXS gateway is required to connect an FXO device for a VoIP call. And an FXO gateway is necessary to connect analog phone lines to VoIP.

Contact Hubtech Limited for any queries and any assistance with your office telephone system.

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