If you’re working with phones, PBX systems, or VoIP devices, you may have heard about FXS and FXO ports. These two are often confusing, especially for beginners in telecom. But once you understand the basic concept, the difference is actually quite simple. This guide will help you clearly understand the difference between FXS and FXO, how they work, and where you will find them in real-life telephone or VoIP systems. We’ll also explain the meaning behind the terms Foreign Exchange Subscriber (FXS) and Foreign Exchange Office (FXO).
What is FXS (Foreign Exchange Subscriber)?
FXS stands for Foreign Exchange Subscriber. This is the port that sends out the telephone service. It provides the dial tone, the voltage that rings the phone, and the power needed to talk over a regular telephone line.
An easy way to remember: FXS is the source of the telephone line. It is like the wall jack in your home or office that you plug your phone into. That wall jack gives you a dial tone. It is connected to something like a PBX or VoIP device, which has an FXS port.
The FXS port sends the necessary signals to your phone so it can ring when there is an incoming call, and so you can hear a dial tone when you pick up the phone. It also sends your voice back and forth during a conversation.
You can find FXS ports on devices like VoIP gateways, analog telephone adapters (ATAs), and some types of routers or IP PBX systems. These ports are made to connect directly to traditional telephones, fax machines, or other analog devices.
FXS vs FXO Difference
What is FXO (Foreign Exchange Office)?
FXO stands for Foreign Exchange Office. This is the port that receives the telephone service from an FXS port. In simple words, the FXO port is what you find on your phone, fax machine, or analog modem. These devices are waiting to get a dial tone, and the FXO port connects them to the telephone line.
When you plug a regular telephone into the wall, you are connecting its FXO port to the FXS port in the wall jack. The FXO port listens for the dial tone and ringing voltage that is sent from the FXS port. When you make a call, the FXO port sends the digits you dial to the FXS, which then connects the call.
You can also find FXO ports on devices like legacy PBX systems, analog phones, fax machines, or VoIP FXO gateways. In VoIP setups, FXO gateways are used to connect your VoIP network to the PSTN (public telephone network), so people can make and receive calls through traditional phone lines.
FXS FXO E1 Multi-service IP PBX System
VoIP FXO gateways
FXS vs FXO: What’s the Real Difference?
The easiest way to understand the difference between FXS and FXO is to remember this:
FXS sends the phone service (dial tone, ring signal, power)
FXO receives the phone service (it expects a dial tone and can make or answer calls)
You can think of FXS as the “provider” and FXO as the “receiver”. FXS PROVIDES the phone line service (power, dial tone, ring), while FXO CONSUMES or USES that service.
Here’s a simple real-world example. Let’s say you have an analog phone and a VoIP gateway:
The analog phone has an FXO port, because it needs to receive the dial tone.
The VoIP gateway has an FXS port, because it gives the dial tone and power.
You connect the FXO port (on the phone) to the FXS port (on the gateway), and the phone now works like it’s connected to a normal phone line.
How Do FXS and FXO Work Together?
FXS and FXO always work as a pair. You need both in a phone system. One sends the signal (FXS), and one receives the signal (FXO).
For example, let’s say you are using a VoIP adapter to connect your old telephone to an internet-based phone system. The VoIP adapter has an FXS port. When you plug your phone into it, the phone gets a dial tone, and you can start making calls using your VoIP service.
In another situation, let’s say your office has a traditional PBX phone system with FXO ports. You want to connect this PBX to the PSTN（Public Switched Telephone Network）. You need a device with FXS ports to connect the PBX to the public network, or you can use a VoIP FXO gateway to convert the analog signal into digital VoIP calls.
FXS and FXO must always be used in the right order—FXO connects to FXS, and not the other way around. If you connect two FXS ports or two FXO ports together, the system won’t work correctly.
Pro Tips for Setting Up FXS and FXO Devices
- Label Your Ports: If you’re managing a lot of cables or devices, always label which ports are FXS and which are FXO. This helps prevent connection mistakes.
- Use FXO Gateways with PSTN Lines: If your company still uses PSTN phone lines, use an FXO gateway to connect those lines to your VoIP system. This lets you make calls over the internet while still using the traditional phone network when needed.
- FXS Gateways for Analog Phones: If you have many old analog phones that you want to use with your VoIP system, get a VoIP gateway with FXS ports. Each FXS port can connect to one phone or fax.
- Test Ring Voltage: Not all phones work well if the ring voltage is too low. Make sure the FXS port provides enough voltage (usually around 90V AC) so the phone rings properly.
- Use Multi-Port Devices: For businesses, multi-port FXS or FXO gatewayshelp manage multiple phone lines without the need for many separate adapters.
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【Edited in July.2025】
