Q.850 cause 18 No user responding

What Q.850 cause 18 means

Nothing came back from the far end at all — no ringing, no answer, just silence until a timer expired. That is what separates it from cause 19, where the phone did ring. Usually an unregistered device, a dead route, or signalling being dropped by a firewall. SIP equivalent is 408 Request Timeout.

Normal class
Sent by The originating switch or your own PBX, on a timer
Occasional

What Q.850 cause 18 No user responding actually means

The call setup message was sent and the sender waited for the destination to say something — anything. It expected either an alerting indication meaning the phone is ringing, or a connect meaning it has been answered. Neither arrived within the allowed time, so the call was abandoned. The critical point for diagnosis is that the destination never confirmed it had even received the call. Under RFC 3398 this becomes SIP 408 Request Timeout, and in a SIP trace it typically corresponds to an INVITE that got no 180 Ringing and often no response whatsoever.

This cause is used when a called party does not respond to a call establishment message with either an alerting or connect indication within the prescribed period of time allocated.
— ITU-T Q.850 section 6.2.7.1.13

The SIP equivalent of Q.850 cause 18

A gateway translating this cause into SIP reports it as SIP 408 Request Timeout. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 18 on a business phone system

The destination device is not registeredA phone that has lost its registration is invisible to the platform. Depending on how the system is built, calls to it may be attempted anyway and simply time out with nothing responding.
Signalling is being dropped in the network pathA firewall closing UDP state, a NAT device losing its mapping, or SIP ALG mangling headers can all mean the INVITE arrives and the reply never gets back. Both ends behave correctly and the call still dies.
A route pointing at equipment that is goneAn IP address, hostname or SRV record that no longer resolves to anything live produces exactly this: messages sent into the void and a timer expiring. Common after a site move or a broadband change.
The far-end SBC or gateway has stopped respondingWhere the destination is another business with its own on-premise system, a hung SBC will accept the packets at network level while never generating a SIP response. From your side it is indistinguishable from a black hole.
An overloaded system failing to respond in timeUnder heavy load some platforms queue call setup rather than rejecting it, and the response arrives after the caller has given up. Correlate failures against time of day to spot this.

How to fix Q.850 cause 18

Check the registration state firstOn the PBX or provider portal, confirm the destination extension or trunk is registered and when it last renewed. An unregistered endpoint explains the symptom completely and takes moments to verify.
Look for a 100 Trying and a 180 Ringing in the traceIf neither appears, nothing acknowledged your INVITE and the problem is reachability. If a 100 appears but no 180, the far end received the call and then stalled, which shifts suspicion to their equipment.
Turn off SIP ALG and check firewall timeoutsRaise the UDP session timeout above the registration interval and disable any SIP helper on the router. Then retest — if responses start arriving, the middlebox was the cause.
Verify DNS and SRV records for the trunkResolve the carrier hostname from the PBX itself, not from a laptop. Systems can hold a stale cached address long after the record has changed, and only a restart or cache flush clears it.
Compare against a known-good destinationCall a number you are certain works from the same extension. If that succeeds, the fault is specific to the destination; if it fails too, your own signalling path is broken and that is where to look.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about Q.850 cause 18

Is cause 18 the same as nobody answering?

No, and the difference is the whole point of having two cause values. Cause 19 means the phone rang and a person chose not to pick up. Cause 18 means the phone never rang, because the far end never responded at all. One is a human outcome, the other is a technical failure.

Why does cause 18 appear after a consistent delay?

Because it is generated by a timer rather than by an event. The switch waits a fixed period for an alerting or connect indication and gives up when it expires. The consistency of the delay is confirmation that nothing responded, not a clue in itself.

Can cause 18 be my own network rather than the far end?

Very often, yes. If your firewall or router is dropping the return signalling, the far end may be ringing perfectly well while you see silence. Test from a connection outside your office network to separate the two possibilities.

Related Q.850 cause values

Source. Q.850 cause 18 No user responding is defined in ITU-T Q.850 section 6.2.7.1.13 and registered in the ITU-T Recommendation Q.850 (10/2018). The causes and fixes above are drawn from our own experience supporting UK business phone systems.

All Call hangup causes (Q.850) codes