Q.850 cause 41 Temporary failure

What Q.850 cause 41 means

A transient network fault stopped this call, and Q.850 is explicit that it is not expected to last: trying again almost immediately will often just work. One-off cause 41s are ordinary background noise on any trunk. What deserves attention is a cluster of them at the same time of day, on one route, or from one site.

Resource unavailable class
Sent by Any switch or gateway in the path, most often the carrier
Common

What Q.850 cause 41 Temporary failure actually means

The network momentarily could not complete the call but expects to recover shortly, so the correct response is another attempt rather than a fault report. Q.850 pairs this deliberately with cause 38: same class, same SIP translation, opposite advice. Under RFC 3398 a gateway renders it as 503 Service Unavailable, and many SIP devices treat a 503 as a signal to try the next route in their list automatically, which is why users often never notice a single occurrence.

This cause indicates that the network is not functioning correctly and that the condition is not likely to last a long period of time; e.g., the user may wish to try another call attempt almost immediately.
— ITU-T Q.850 section 6.2.7.2.5

The SIP equivalent of Q.850 cause 41

A gateway translating this cause into SIP reports it as SIP 503 Service Unavailable. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 41 on a business phone system

A brief wobble on the carrier networkRouting protocols reconverging, a node restarting or a link flapping can make a call fail while everything around it succeeds. By the time you look, the condition has usually cleared.
A failover happening at the exact moment you dialledWhen a session border controller or trunk fails over between nodes, calls in flight during the handover are lost. Calls placed a few seconds either side are unaffected, which makes the failure look random.
Packet loss on the signalling pathA lost INVITE or a lost reply can produce a failure even when audio quality on completed calls is perfect. This is worth stating plainly, because people tend to test call quality and conclude the line is fine.
A momentary capacity pinchA short spike that clears in seconds can be reported as a temporary failure rather than as congestion. If the spikes get longer and more frequent, the underlying issue is capacity and you will start seeing cause 42 instead.
Overnight maintenance on the carrier platformPlanned work often produces scattered failures during a change window. Failures that cluster between about midnight and 6am, then stop, usually need no action beyond noting the pattern.

How to fix Q.850 cause 41

Redial once before treating it as a faultThis sounds too simple to write down, but it is what the specification actually recommends for this cause. If the second attempt connects, you have your answer and no ticket is needed.
Measure the rate before escalatingCount cause 41 records against total outbound attempts for the same period. A handful in several thousand calls is normal; several per hundred is a real problem, and the number is what turns a vague complaint into a diagnosable ticket.
Look for a pattern in time, route or siteSort the failures by hour, by destination prefix and by originating extension. A pattern that survives all three cuts points squarely at whichever element they share.
Check the signalling path specifically, not just call qualityTest for loss and jitter on the link while calls are in progress, and check whether signalling and media take the same path. Signalling problems can hide behind perfectly good audio on the calls that do connect.
Configure a secondary route so retries are automaticA second trunk or a fallback carrier means the PBX handles the retry without the user knowing. You will still want to watch the failure rate, but nobody has to redial by hand.

Fixing the underlying problem

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

Questions about Q.850 cause 41

Is cause 41 my broadband or the carrier?

Either can produce it, and the way to tell them apart is scope. If several sites on different connections see it at once, it is upstream; if only one site does, look at that site's link and its signalling path first.

How many cause 41 records are normal?

There is no official threshold, but a very small fraction of attempts on a healthy trunk is unremarkable and rarely noticed by users. The useful test is whether the rate is stable over time, because a rate that is climbing matters even while it is still low.

Why do cause 41 and cause 38 both appear as SIP 503?

RFC 3398 maps several resource-unavailable causes onto the same SIP response, so detail is lost at the gateway. If you only have the SIP side of the call you cannot tell them apart, which is a good reason to keep the Q.850 cause in your call records.

Related Q.850 cause values

Source. Q.850 cause 41 Temporary failure is defined in ITU-T Q.850 section 6.2.7.2.5 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