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What do the terms FXS and FXO mean?

FXS and FXO are the name of ports used by Analog phone lines (also known as POTS - Plain Old Telephone Service) or phones.

FXS - Foreign eXchange Subscriber interface is the port that actually delivers the analog line to the subscriber. In other words it is the ‘plug on the wall’ that delivers a dialtone, battery current and ring voltage.

FXO - Foreign eXchange Office interface 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 phone, the device is often called the ‘FXO device’.

FXO and FXS are always paired, i.e similar to a male / female plug.

Without a PBX, a phone is connected directly to the FXS port provided by a telephone company.

fxs/fxo without a pbx
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 to).

fxs/fxo with a pbx
FXS / FXO with a PBX


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.

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 different types that 3CX Phone System 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 Phone System for Windows automatically configures FXS/FXO Gateways to allow you to easily continue using your existing PSTN lines and/or analog phones. You can download the Free edition here

More information about FXS / FXO and VoIP in general can be found in our SIP / VoIP Video tutorials, 'Voip Nuggets'. VoIP Nuggets are short youtube technical training tutorials about VoIP & SIP. Click here for the latest list of VoIP Nuggets.


FXS/ FXO procedures – how it technically works

If you are interested to know in more technical detail how an FXS/ FXO port interoperate, 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.

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