# FXO

> 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.

- Also known as: Foreign Exchange Office, FXO port
- Last reviewed: July 29, 2026

## 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

**Claim:** An FXO port can power a telephone.

**Correction:** FXO draws power from the line rather than supplying it. Powering a telephone requires an FXS port.

## Frequently asked questions

**Is FXO or FXS what I need for POTS replacement?**

FXS. Your analog equipment expects to be fed dial tone, so the replacement appliance must present FXS ports to it.

## Sources

- [Copper Retirement](https://www.fcc.gov/modernizing-telecommunications-networks-what-government-officials-need-know) — Federal Communications Commission


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Canonical page: https://dataremote.com/newsroom/definitions/fxo
Organization: DataRemote, Inc. — POTS line replacement, cellular failover, and fixed wireless access solutions. https://dataremote.com