What do the Terms FXS and FXO Mean?

What do the Terms FXS and FXO Mean?

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 expression “POTS” was initially intended as a joke but is now used as a common expression in the telecommunications industry.

FXS (Foreign Exchange Subscriber) is the port that actually delivers the analog line to the subscriber. In other words, it is the “plug in the wall” that delivers a dial tone, battery current and ring voltage. This is the jack or interface to the phone system which FXO devices can be connected to.

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).

FXO and FXS are always paired, i.e similar to a male / female plug. If no phone system is used the telephone (FXO) is directly connected to the FXS port. The port is provided by the telephone company.

fxs/fxo without a pbx

If you have a PBX, then you connect the lines provided by the telephone company to the PBX and then the phones to the PBX. Therefore, the PBX must have both FXO ports (to connect to the FXS ports provided by the telephone company) and FXS ports (to connect the phone or fax devices). Many traditional phone systems provide both, FXS and FXO ports while many IP Phone systems require additional gateways to provide FXS or FXO ports.

fxs/fxo with a pbx

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 will come across the terms FXS and FXO when deciding to buy equipment that allows you to connect analog phones to a VoIP Phone System or traditional PBXs to a VoIP service provider or to each other via the Internet.

If you want to connect an analog fax machine to the phone system, you will need at least one FXS port. If you want to use analog telephone lines with your phone system, you need one FXO port per line.

An 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. There are a number of different FXO gateways available. You can view a list of gateways that 3CX supports here.

FXO gateway

An 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 (which normally are connected to the telephone company) to the Internet or a VoIP system.

FXS gateway

An FXS adapter a.k.a. ATA adapter

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 (ATA) adaptor

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 Training Videos.


FXS/FXO Procedures – How Does it Work in Detail

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:

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

Inbound call:

  1. The FXS port receives a call, and then sends a ring voltage to the attached FXO device.
  2. The phone rings.
  3. 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.

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.

Learn more about an office phone system or small business phone system and break free from your proprietary PBX.

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