# Calling Party Control

> Calling Party Control (CPC) is the disconnect signal on an analog line: a brief drop in loop current from the serving equipment that tells the premises device the far party has hung up, so it can release the line.

- Also known as: CPC, CPC signal, disconnect supervision, loop-current disconnect
- Last reviewed: July 30, 2026

## How it works

When the far end of a call disconnects, the serving switch (or the FXS port emulating one) momentarily interrupts loop current — commonly for a few hundred milliseconds. Equipment watching the line, such as voicemail systems, auto-attendants, PBX trunks, and some alarm dialers, treats that interruption as the definitive hang-up signal and releases the line for the next call.

## Why it matters

Without a proper CPC signal, automated equipment can hold a line open long after a caller hung up — voicemail recording minutes of dial tone, a trunk stuck busy, a panel waiting on a dead call. A POTS replacement's FXS ports need to generate disconnect supervision the attached equipment recognizes, with timing it accepts.

## Common use cases

- Voicemail and auto-attendant systems releasing lines promptly
- PBX analog trunks clearing after the caller hangs up
- Automated dialers detecting the end of a call

CPC is invisible while it works and maddening when it doesn't — stuck lines and phantom busy
trunks after a line migration are the classic symptom.

## Diagnosing a stuck line

CPC problems present as a line that will not release. The symptom set is distinctive once
recognized: a fax machine that reports a busy far end on every attempt, an alarm panel that
cannot seize its line to dial, a device that holds a call open indefinitely after the other
party hung up, or a port that appears permanently off-hook.

The reason is that many analog devices have no other way to learn a call ended. Without a
disconnect signal, the device waits — sometimes until a timeout that is minutes long, sometimes
until power-cycled.

To diagnose, place a call from the endpoint to a number you control, hang up the far end
first, and watch whether the endpoint releases within a couple of seconds. If it does not, the
disconnect signal is either absent or outside the duration the endpoint recognizes.

Both the presence and the length of the signal matter — a device expecting a longer break may
ignore a shorter one entirely, which looks identical to no signal at all.

## Common misconceptions

**Claim:** All analog equipment detects hang-up the same way.

**Correction:** Some devices rely on CPC, others on tone detection or timeouts, and the loop-current interruption they expect varies in duration. Matching disconnect behavior to the specific equipment is part of endpoint compatibility testing.

## Frequently asked questions

**Do POTS-replacement appliances provide CPC?**

FXS ports on replacement appliances commonly generate a configurable loop-current disconnect signal. Whether a particular device recognizes it depends on that device's requirements — confirm per endpoint and configuration.

## Sources

- [ATIS Telecom Glossary](https://glossary.atis.org/) — Alliance for Telecommunications Industry Solutions


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