Failing calls: how to diagnose them by SIP code and Q.850 cause
When the calls on a SIP trunk or an 0800 start to fail, the SIP code and Q.850 cause of the first failed call say where it stopped and whose next step it is. This guide gathers the codes that show up most, 503, 486, 480, 487 and 603, and the causes 16, 34 and 127, with the right question to ask for each.
Summary
- The SIP code says what the server decided to do with the call; the Q.850 cause, in the Reason header, says why, and it is almost always what the carrier uses to locate the fault.
- SIP 503 with cause 34 is a lack of channels or routes, and SIP 486 with cause 17 is usually the simultaneous channel limit being exhausted.
- SIP 480 is a destination that is unregistered or unavailable, SIP 487 is a call canceled before it was answered and SIP 603 is an explicit decline.
- Cause 16 is normal clearing and not an error; cause 127 is a reason the network did not report.
- To open a ticket with the carrier, gather the time with time zone, the number called, the SIP code, the Q.850 cause and the Call-ID of each attempt.
Where do you start when calls fail?
With the code of the first call that failed, not with the complaint. Each rejected call returns a SIP code, and many also return a Q.850 cause in the Reason header (RFC 6432). Together they say where the call stopped and who can fix it.
A short path, in order:
- Gather the attempts: time with time zone, origin number, number called, SIP code, Q.850 cause and Call-ID of each one.
- See whether the call reached your SBC or PBX. If none did, the failure is before you, on the carrier’s route.
- Read the code in the table below and follow its page for the detail.
- Repeat from other origins: a landline and mobile phones on different carriers show whether the failure applies to all of them.
A scheduled test call helps a lot here: it already brings the time, the code and the cause of each attempt, including the ones nobody saw. See what is synthetic call monitoring.
What does each SIP code mean?
| Code | What it means | Most common cause on a trunk or 0800 | Next step |
|---|---|---|---|
| 503 Service Unavailable | The server is temporarily unable to process the call | Lack of channels or routes at the carrier, or an SBC that is down | Read the Retry-After and the Q.850 cause; open a ticket with the Call-ID if it persists |
| 486 Busy Here | The destination was reached but cannot accept another call | Simultaneous channel limit exhausted | Compare the active calls at that moment with the contracted limit |
| 480 Temporarily Unavailable | The destination was located but is not available now | Destination unregistered, on Do Not Disturb or not answering | Check the destination’s registration and the Retry-After |
| 487 Request Terminated | The request was ended by a CANCEL before it was answered | The caller hung up while it rang, or the PBX gave up on a timeout | Not a destination failure; investigate only when excessive |
| 603 Decline | The destination was contacted and explicitly declined the call | The person rejected it, or a call block | See whether the decline came from the destination, the PBX or the carrier |
For the other codes, the SIP code index has a page for each, and the SIP code lookup opens the code you type.
And the Q.850 causes?
The Q.850 cause is the reason the network gives for ending the call, inherited from traditional telephony and carried by SIP in the Reason header. The three that raise the most questions:
| Cause | What it indicates | When to investigate |
|---|---|---|
| 16, Normal call clearing | Normal clearing: one side hung up after the call was established | Only when many calls end with 16 within a few seconds, with no audio or right after the IVR answers |
| 34, No circuit/channel available | There was no free circuit or channel to complete the call | Always: it is usually the channel limit exhausted, yours or the carrier’s |
| 127, Interworking, unspecified | The call crossed an interconnection that does not report the reason | Find out where the call failed and what response came from there |
The Q.850 cause index lists the rest.
How does the SIP code connect to the Q.850 cause?
The two fit together. SIP 503 often appears with cause 34 (no channel available), but also with others, such as 38, 41 and 88. SIP 486 usually comes with cause 17, destination busy. SIP 603 goes with 21, call rejected. SIP 480 comes with causes 19, 20 and 31, and SIP 487 has no associated cause, because it is a cancellation. The mapping between the two families is in RFC 3398.
Which failure patterns deserve attention?
- Many 503s with cause 34 at a peak hour: trunk capacity exhausted. Compare the peak of simultaneous calls with the contracted channels.
- 486 and cause 17 on an IVR or 0800: usually the same channel limit, showing up as “busy”.
- 480 on an extension or an IVR that should answer: destination unregistered. Check the SIP registration.
- Many 487s in a row: the PBX is giving up before the call is answered, or callers hang up while it rings. See how many seconds passed before the CANCEL.
- Cause 16 on very short calls: the clearing is normal, but the problem may be earlier. See who sent the BYE and whether there was audio both ways.
- Cause 127 on calls that cross a gateway: the reason was lost in the conversion between networks. Ask the carrier for the originating code.
What should you take to the carrier?
A table, one row per attempt: time with time zone, origin number, number called, SIP code, Q.850 cause, Call-ID and what was heard. That is what the carrier can locate in its own records. The guide How to enforce a carrier SLA with proof shows how to turn these records into a claim.
How does Monitor by 2N5 use these codes?
Each Monitor by 2N5 test call records the SIP code and Q.850 cause returned, and the four failures in a row that open an incident reach the alert with that evidence. See SIP trunk monitoring and 0800 and IVR monitoring.
Frequently asked questions
Why are my calls failing on the SIP trunk?
Look at the SIP code of the first failure and the Q.850 cause in the Reason header. A 503 with cause 34 points to a lack of channels or routes; a 486, to the channel limit; a 480, to an unregistered destination; a 603, to an explicit decline. Without the code, any diagnosis is a guess. Record the time, the number called and the Call-ID of each attempt.
What does SIP 503 Service Unavailable mean?
SIP 503 Service Unavailable means the server is temporarily unable to process the call, because of overload or maintenance. On trunks, the most common reasons are the carrier running out of channels or routes, or an SBC being down. It matters when it repeats: read the Retry-After and the Q.850 cause and open a ticket with the Call-ID.
Is Q.850 cause 16 an error?
No. Cause 16, Normal call clearing, is the normal end of a call: one side hung up after the call was established. It is the most common value in call logs. It only deserves investigation when many calls end with 16 within a few seconds, with no audio or right after the IVR answers, because then the normal clearing may be a consequence of another problem.
What is the difference between the SIP code and the Q.850 cause?
The SIP code is the protocol's response, such as 503 or 486, and has classes: 4xx is a request failure, 5xx a server failure and 6xx a global failure. The Q.850 cause comes from traditional telephony and gives the reason more precisely, such as 34 for no channel available. RFC 6432 defines how the cause travels in the Reason header.
How do I know whether the problem is the carrier's or my network's?
See whether the call reaches your SBC or PBX. If none does, the failure is before you, on the carrier's route, and the code and time of each test call are the proof for the ticket. If it arrives and your inbound rule rejects it, the problem is in your own configuration.
What does SIP error 503 / 486 / 480 / 408 mean?
503 means service unavailable: on trunks, usually no channel or no route at the carrier, or an SBC that is down. 486 means busy, normally the channel limit. 480 means the destination is temporarily unavailable, such as a number that is not registered. 408 means timeout: the call got no answer, because of the route or a firewall. Always read the code together with the Q.850 cause and the time of each call.
What is Q.850 cause 16 / 34 / 127?
A Q.850 cause is the disconnect reason defined by traditional telephony, carried in the Reason header of the SIP response. Cause 16 is normal call clearing, not an error. Cause 34 is no circuit or channel available. Cause 127 is interworking, unspecified: the call crossed a network that does not report the reason, and the fault can be on any leg.