Q.850 cause 16 Normal call clearing

What Q.850 cause 16 means

Not a fault. Cause 16 means somebody hung up — it is the normal ending for a successful call and the most common value in any CDR. Only investigate the pattern around it: calls ending at two seconds, or always at the same duration, point to something else. The cause itself is the system working correctly.

Normal class
Sent by Whichever end hung up first, which is usually a person
Common

What Q.850 cause 16 Normal call clearing actually means

One of the two parties asked for the call to be cleared, and the network obliged. The specification is explicit that under normal conditions the source of this cause is not the network — it is a user action being reported faithfully. In SIP terms there is no failure response involved at all: RFC 3398 associates this cause with a BYE or CANCEL rather than an error status code, which is exactly why you will never see a matching 4xx or 5xx in the trace. If most of your CDR rows carry cause 16, that is what a healthy phone system looks like.

This cause indicates that the call is being cleared because one of the users involved in the call has requested that the call be cleared. Under normal situations, the source of this cause is not the network.
— ITU-T Q.850 section 6.2.7.1.11

The SIP equivalent of Q.850 cause 16

RFC 3398 maps this cause to a SIP BYE or CANCEL request rather than to a response code. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 16 on a business phone system

Somebody put the phone downThe overwhelming majority of cause 16 records. The conversation finished and one party ended it. There is nothing here to diagnose, and no configuration change will make it stop appearing.
An automated system ended the call deliberatelyVoicemail finishing a message, an auto attendant timing out after no key press, or a call recording platform completing a leg all clear the call normally. The CDR looks identical to a human hanging up.
A session timer expiring mid-callSIP session timers refresh a call periodically. If the refresh does not get through, one end tears the call down with a BYE and it is logged as normal clearing. This is the classic cause of calls that always die at the same duration.
A firewall or SBC closing the sessionMiddleboxes that time out UDP state, or SIP ALG rewriting signalling incorrectly, can cause one end to send a perfectly well-formed BYE. The clearing is normal even though what prompted it was not.
A transfer or divert completingWhen a call is transferred, the original leg is cleared normally as the new one is established. Reading a single leg in isolation makes a completed transfer look like a hang-up.

How to fix Q.850 cause 16

Read the duration before anything elseCause 16 on a four-minute call needs no action. Cause 16 on a two-second call, repeatedly, means people are hanging up immediately — which is a call quality, greeting or routing question rather than a signalling one.
Identify which end sent the BYEThe CDR or trace will show the direction. If your own system is consistently the one clearing calls that users say they did not end, the problem is inside your network and worth pursuing.
Check session timers if calls die at a fixed lengthCalls ending at almost exactly the same duration every time is the signature of a timer, not a person. Compare the session expiry configured on the PBX with what the carrier expects and align them.
Disable SIP ALG and check NAT keepalivesOn long calls with little traffic in one direction, a router can drop the signalling path. Turning off SIP ALG and enabling keepalives on the trunk resolves a large share of unexplained mid-call clearing.

Fixing the underlying problem

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

Questions about Q.850 cause 16

Is Q.850 cause 16 an error?

No. It is the standard, correct way of recording that a call ended because someone ended it. People search for it because it appears next to a call that felt like a failure, but the cause value itself is confirming the system behaved properly.

Why do all my calls end with cause 16?

Because that is what a working phone system produces. Every successfully completed call clears normally. It would be far more concerning to see a mixture of other causes, since those describe calls that did not end by choice.

Our calls drop after the same number of minutes with cause 16 — why?

A consistent duration means a timer, not a person. Look at the SIP session timer configuration on your PBX and trunk, and at UDP timeouts on the firewall. Something is deciding the session has expired and clearing it politely.

Related Q.850 cause values

Source. Q.850 cause 16 Normal call clearing is defined in ITU-T Q.850 section 6.2.7.1.11 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