Q.850 cause codes are the numbers, defined in ITU-T Recommendation Q.850, that telephone networks use to say why a call ended or failed: 16 is normal call clearing, 17 user busy, 1 unallocated number, 34 no circuit available. VoIP switches record them in CDRs and carry them in the SIP Reason header, so they are the most precise clue when a route fails.
What are Q.850 cause codes?
Q.850 defines the cause values used by ISDN and ISUP signalling (the SS7 protocol between telephone exchanges). When a call crosses from SIP into the traditional network, or back, the gateway translates between Q.850 causes and SIP response codes. RFC 3398 gives the standard mapping, and RFC 3326 lets a SIP message carry the original cause in a Reason header, for example Reason: Q.850;cause=34.
Causes are grouped in ranges: 1 to 31 normal events, 32 to 47 resource unavailable, 48 to 63 service or option not available, 64 to 79 service or option not implemented, 80 to 95 invalid message, 96 to 111 protocol error, 112 to 127 interworking.
Q.850 cause codes list (most common)
| Cause | Q.850 name | Meaning | Usual SIP code (RFC 3398) |
|---|---|---|---|
| 1 | Unallocated (unassigned) number | Number is valid in format but not assigned | 404 |
| 16 | Normal call clearing | One party hung up after the call | BYE or CANCEL |
| 17 | User busy | Called party cannot accept another call | 486 |
| 18 | No user responding | No alert or answer from the called side in time | 408 |
| 19 | No answer from user (user alerted) | Phone rang but nobody answered in time | 480 |
| 21 | Call rejected | Called equipment refused the call | 403 (603 if user-side) |
| 27 | Destination out of order | Interface to the destination not working | 502 |
| 31 | Normal, unspecified | Normal event with no more specific cause | 480 |
| 34 | No circuit/channel available | No circuit free to carry the call | 503 |
| 38 | Network out of order | Network fault likely to last; retry unlikely to work | 503 |
| 41 | Temporary failure | Short-term network fault; retry may work | 503 |
| 42 | Switching equipment congestion | Switch under heavy load | 503 |
| 44 | Requested circuit/channel not available | The specific circuit asked for cannot be used | Not mapped |
| 47 | Resource unavailable, unspecified | Some resource missing, no specific cause | 503 |
| 88 | Incompatible destination | Destination cannot handle the call type | 503 |
| 102 | Recovery on timer expiry | A protocol timer expired during setup | 504 |
| 127 | Interworking, unspecified | Cause lost when crossing another network | 500 |
For the SIP side of the same failures, see our SIP error codes list.
What does cause code 16 mean?
Cause 16, normal call clearing, means one of the parties ended the call. It is the code you want to see on most answered calls. A high share of cause 16 on calls lasting only a few seconds is still worth investigating, because it may hide short duration calls or false answer supervision.
What do cause codes 17, 18 and 19 mean?
These are user-side results. 17 is busy. 18 means the called side did not even confirm alerting within the network timer, often an unreachable mobile. 19 means the phone rang but was not answered. On a healthy route their shares are stable per destination. They map to SIP 486, 408 and 480.
What do cause codes 31, 34 and 41 mean?
31 is a catch-all for normal events; some switches use it far too often, which makes it hard to read. 34 (no circuit available) and 41 (temporary failure) are network-side: the route ran out of capacity or hit a fault. Together with 38, 42 and 47 they are mapped to SIP 503 Service Unavailable, and a rising share is the classic sign of a congested or failing supplier.
How do you use cause codes to troubleshoot routes?
- Group failures by cause per destination and per supplier, every day, from your CDRs.
- Separate user causes (16, 17, 18, 19, 21) from network causes (27, 34, 38, 41, 42, 47, 102). Network causes are the supplier's problem; this is the logic behind NER.
- Watch cause 1 and 28 (invalid number format): they usually mean a dialplan or prefix problem on your side.
- Treat 127 with care: the real cause was lost on the way, so compare with another route before blaming one.
When network causes stay high on one supplier, VoIPMatcher can source a replacement VoIP termination route from its carriers and manage it for you. Request a VoIP route.
Frequently asked questions
What is the difference between Q.850 and Q.931?
Q.931 is the ITU-T recommendation for ISDN user-network signalling, the messages that set up and clear calls on a PRI line. Q.850 defines the cause values and location field that Q.931 and ISUP messages carry. In other words, Q.931 is the protocol, and Q.850 is the shared list of reasons a call ends.
What is the Reason header in SIP?
The Reason header, defined in RFC 3326, lets a SIP message carry the reason a call ended in another protocol. A typical example is Reason: Q.850;cause=17;text="User busy". Gateways add it so the original telephone network cause survives the translation to SIP, which makes CDR analysis much more accurate.
Is cause code 31 an error?
Not necessarily. Cause 31, normal unspecified, is a catch-all for normal events that have no more specific cause. Some switches use it for unanswered or rejected calls, others for almost every failure. If it is a large share of your failed calls, ask the supplier how they map it and compare with another route.
What is Q.850 cause 0?
Cause 0 is not a defined Q.850 cause. When it appears in a CDR, the switch usually did not receive or record a cause, for example because the call was cleared internally or the far end sent none. Treat it as unknown and look at the SIP response code and timing to understand what happened.
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