How to read the Hangup Cause Report and find why calls drop

Calls drop and you want to know whose fault it is. The Hangup Cause Report answers that directly: it shows the carrier hangup causes for all calls going out from your systems, and it lets you define only specific hangup causes to display. That filter turns a noisy list into a focused investigation.

What a hangup cause is

A Hangup cause is the numeric reason a call ended, returned by the carrier when the call tears down. It is not the same as the call Disposition your agent picks, and it is not quite the same as the SIP response code from call setup — the hangup cause is specifically the teardown reason. Common ones tell a clear story: cause 16 is a normal clearing, cause 17 is busy, cause 21 is call rejected, cause 1 is unallocated number.

Filtering to the cause you care about

The filter is the feature. Instead of reading every cause, define the specific ones to display:

• Filter to cause 21 (call rejected) to see whether your Carrier or the far end is actively refusing calls — often a sign of a flagged CID (caller ID).
• Filter to cause 1 (unallocated number) to catch bad data — dead numbers in your Lead lists inflating your failure rate.
• Filter to causes tied to CONGESTION to see network or capacity pressure on the Trunk.

From cause to cure

flowchart TD
  A[Open Hangup Cause Report] --> B{Filter to suspect cause}
  B --> C{Which cause dominates?}
  C -->|Cause 1 unallocated| D[Clean lead data]
  C -->|Cause 21 rejected| E[Check CID reputation]
  C -->|Congestion| F[Carrier capacity / trunk]
  C -->|Cause 16 normal| G[Not a fault: normal clearing]

The reflex to resist is treating every hangup as a problem. Cause 16, normal clearing, is what a healthy completed call looks like. The drops worth chasing are the ones that should not be happening: rejections on numbers that used to connect, congestion that appeared with no change on your side, unallocated numbers in a list you trusted.

Once you have a dominant cause and a window, pull a few of those calls by Asterisk uniqueid and follow them through your other logs to confirm the carrier really is the source before you escalate. Causes also drift in meaning between carriers — one carrier's cause 38 for a network problem may surface as plain congestion on another — so anchor your reading in what your own carrier documents, not in a generic table you found somewhere. The numbers are a starting point for the conversation, not the final verdict.

Run this report on a schedule and the value compounds. A baseline of normal cause distribution makes anomalies obvious: when cause 21 jumps from two percent to twenty overnight, you do not have to wonder whether it is normal — you already know it is not. That baseline is the difference between catching a Carrier degradation the same day and finding out a week later when results have already slipped.

Where to go next

Pair this with the Carrier Log Report to get the downloadable attempt-and-response log for your ticket, and the Dial Log Report for the per-call SIP cause column. If rejections trace back to your numbers, the Caller ID Log Report shows which CIDs you have burned. The troubleshooting playbook puts these in order.

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