Q.850 cause 31 Normal, unspecified

What Q.850 cause 31 means

The catch-all for the normal class: the event was ordinary, but the equipment had nothing more specific to say. It usually means detail was lost in translation between networks, so treat it as a prompt to get the trace rather than an answer in itself. A gateway renders it as SIP 480 Temporarily Unavailable.

Normal class
Sent by Almost anything in the path — it is the catch-all of the normal class
Common

What Q.850 cause 31 Normal, unspecified actually means

The specification defines this as reporting a normal event only when no other cause in the normal class applies. In other words, whatever ended the call was not an equipment failure or a congestion condition, but the sender either could not or would not be more precise. In practice you meet it most often where signalling has crossed a boundary and specificity was lost. RFC 3398 maps it to SIP 480 Temporarily Unavailable, which is equally unspecific — so neither value is going to tell you what actually happened. The information you need is elsewhere.

This cause is used to report a normal event only when no other cause in the normal class applies.
— ITU-T Q.850 section 6.2.7.1.25

The SIP equivalent of Q.850 cause 31

A gateway translating this cause into SIP reports it as SIP 480 Temporarily Unavailable. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 31 on a business phone system

A vendor default where nothing better fitsMany platforms fall back to this whenever their internal reason has no clean Q.850 equivalent. It tells you the platform considered the outcome normal, and nothing more than that.
Detail lost crossing a gateway or SBCWhen a specific cause is translated between protocols and back again, precision leaks away. The element that replaced a meaningful cause with this one is the element to question.
An application clearing the call without setting a causeIVRs, voicemail platforms and call recording systems end calls programmatically. Where the developer did not populate a reason, the platform supplies this by default.
A carrier normalising several outcomes into oneSome wholesale platforms deliberately collapse a range of downstream reasons into a single value before passing them to customers. You are seeing their summary, not the original event.
A policy decision the switch chose not to describeRouting and policy engines sometimes end calls for commercial or configuration reasons without explaining themselves. The outcome is normal from their perspective and opaque from yours.

How to fix Q.850 cause 31

Do not attempt to diagnose from the cause value aloneThis cause carries almost no information. Anyone offering a confident explanation based only on seeing it is guessing. Go and get evidence before forming a theory.
Capture a full SIP trace of a failing callThe trace shows the sequence of messages and often carries a Reason header preserving the original cause before translation. That header is frequently the answer on its own.
Find where along the path the cause changedCompare what your PBX logged with what the carrier logged. If one end recorded something specific and the other recorded this, the element between them is where the detail was lost.
Correlate with duration and directionA pattern of very short calls suggests setup problems; long calls dying at consistent durations suggest timers. The surrounding data is more diagnostic than the cause value.
Escalate with a specific example, not a summaryGive your carrier one call: date, time to the second, calling and called numbers, and the Call-ID. They can look up their own internal reason, which will be far more specific than what reached you.

Fixing the underlying problem

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

Questions about Q.850 cause 31

What does cause 31 actually tell me?

Only that the sending equipment classed the outcome as normal rather than a failure, and had no more specific value to use. It is a placeholder. Treat it as a prompt to collect a trace rather than as a diagnosis.

What is the difference between cause 31 and cause 16?

Cause 16 is definite: somebody hung up. Cause 31 is deliberately vague: something normal happened and nobody said what. If you were expecting cause 16 and got cause 31, an intermediate element has probably rewritten it.

Should I raise a fault when I see cause 31?

Only if calls are genuinely failing. If they are, raise it with a specific example call rather than with the cause value, because the value alone gives your carrier nothing to investigate.

Related Q.850 cause values

Source. Q.850 cause 31 Normal, unspecified is defined in ITU-T Q.850 section 6.2.7.1.25 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