Definition
FXO
Also known as: Foreign Exchange Office, FXO port
FXO (Foreign Exchange Office) is the interface that connects to a telephone line and consumes dial tone — the side of the connection that behaves like a telephone.
How it works
An FXO port draws loop current from the line, closes the loop to go off-hook, sends DTMF digits, and detects ring voltage. It is the counterpart to FXS: whatever supplies the line is FXS, and whatever attaches to it is FXO.
Why it matters
Understanding the pairing prevents a common wiring error. When integrating a replacement appliance with an existing phone system, you have to know which side of each connection supplies dial tone and which consumes it, or the link will not come up.
Common use cases
- Connecting a PBX trunk port to an existing analog line
- Integrating legacy phone systems during a phased migration
The rule of thumb: dial tone flows from FXS to FXO. If you can identify which device is expected to supply it, the wiring follows.
Reading a wiring diagram by following dial tone
Most analog wiring confusion resolves by asking a single question at each connection: which side is supposed to supply dial tone? That side is FXS. The side listening for it is FXO.
Worked through: a telephone listens for dial tone, so the jack it plugs into is FXS. A legacy PBX trunk port listens for dial tone from the carrier, so that port is FXO and the carrier side is FXS. A replacement appliance takes the carrier’s place for local equipment, so its ports facing that equipment are FXS.
The failure mode when this is reversed is silence — no dial tone, no ring, and no error message, because neither side is doing anything wrong on its own terms. Both are waiting.
When an appliance needs to connect to equipment that expects to be the network — an older PBX trunk port, for instance — the interface requirement flips, and confirming the port type on both ends before installation avoids a return visit.
Common misconceptions
-
“An FXO port can power a telephone.”
FXO draws power from the line rather than supplying it. Powering a telephone requires an FXS port.
Frequently asked questions
FXS. Your analog equipment expects to be fed dial tone, so the replacement appliance must present FXS ports to it.
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