Foreign Exchange Station (FXS) and Foreign Exchange Office (FXO) are the two complementary kinds of analog telephone ports. An FXS port acts like the phone company’s end of a line: it supplies dial tone, ringing voltage and loop power to an analog device such as a phone, fax machine or alarm dialer. An FXO port acts like the phone’s end: it plugs into a line or extension that already provides dial tone. A working analog connection pairs one of each. FXS is also sometimes expanded as Foreign Exchange Subscriber; it means the same port.
At a glance
- FXS ports supply dial tone and power; analog phones, fax machines, modems and alarm dialers plug into them.
- FXO ports receive dial tone; they connect to a phone line, a PBX extension or another device’s FXS port.
- A working analog connection pairs an FXS port with an FXO port: the phone jack on the wall is effectively FXS, and the phone itself is FXO.
- VoIP gateways and analog telephone adapters commonly use FXS ports to keep analog devices working on IP phone services.
- Port names matter when buying gateways for POTS replacement, connecting older phone systems or keeping backup lines.
What problem it solves
Most businesses still have analog devices long after their desk phones went digital: fax machines, credit card terminals, fire and burglar alarm panels, elevator emergency phones, door entry systems, overhead paging and modems on building equipment. Each of these was built to plug into a plain old telephone service (POTS) line. As carriers retire copper and businesses move to VoIP, those devices need something that behaves like a phone line.
The FXS/FXO naming is how the industry describes which side of an analog connection a port is on. Getting it right is the difference between a gateway that keeps a fax machine or an alarm panel working and one that can’t connect at all. It also tells you what a device can do: a box with FXS ports can host analog phones; a box with FXO ports can use existing lines.
How it works
FXS (station side). An FXS port does what a phone company’s central office used to do for a line. It provides battery voltage that powers a basic phone, generates dial tone when the handset is lifted, sends ringing voltage for incoming calls, detects dialed digits and passes caller ID. A VoIP gateway or analog telephone adapter uses its FXS ports to convert that analog signal into VoIP, usually SIP, and back.
FXO (office side). An FXO port behaves like a phone. It goes off-hook to draw dial tone, dials digits and detects ringing on the line it is plugged into. Gateways and older phone systems use FXO ports to connect to analog lines from a carrier or to analog extensions on another system.
Typical combinations.
- Analog phone or fax → FXS port on a gateway → VoIP service.
- Carrier analog line → FXO port on a gateway or PBX → calls routed through that system.
- Gateway FXS port → FXO trunk port on an older PBX, so the PBX can use SIP trunks without being replaced.
Limits. Analog signaling over VoIP is sensitive to network quality and to compression. Fax often needs a fax-friendly protocol such as T.38 or carefully configured pass-through (see fax over IP). Alarm panels and other dial-up devices can fail in ways that aren’t obvious until an event, so they need testing. FXS ports also have limits on how many devices and how long a cable run they can drive, which vary by product.
When it matters for buyers
- When replacing copper lines. POTS replacement services commonly deliver lines through a device with FXS ports, often with cellular or broadband backhaul and battery backup.
- When keeping an older phone system. FXS-to-FXO connections can bridge a legacy PBX to newer services during a phased migration.
- When life-safety devices are involved. Fire alarm, elevator and emergency phone lines can have specific code and monitoring requirements that vary by jurisdiction; involve the equipment vendor and the authority having jurisdiction before switching them.
- When ordering gateways. Count the analog devices per site and specify the number of FXS and FXO ports, power needs and backup requirements.
- When copper retirement notices arrive. Inventory every device on an analog line, including ones nobody remembers, before the service changes.
Our POTS replacement overview covers options for moving analog lines off legacy copper service.
Questions to ask vendors
- How many FXS and FXO ports does the device have, and can we add more?
- Which devices have you tested on these ports: fax machines, alarm panels, elevator phones, postage meters, card terminals?
- Do you support T.38 for fax, and what alarm communication formats have been validated on your service?
- What happens to these lines during a power or internet outage, and how long does battery backup last?
- Does the service meet the requirements our fire alarm or elevator vendor and local authority have given us?
- Who supports the device on site, and how are firmware and configuration managed?
How it differs from an analog telephone adapter (ATA)
FXS and FXO describe port types, not products. An analog telephone adapter (ATA) is a device, usually small, that converts analog phones to VoIP, and it does that by providing one or more FXS ports. Larger VoIP gateways may offer many FXS ports, FXO ports, or both, and POTS replacement devices typically combine FXS ports with network connectivity and backup power. When comparing ATAs and gateways, the FXS and FXO port counts tell you what each one can connect.