Q.850 cause 34 (No circuit/channel available): what it means

Short answer: Q.850 cause 34 (No circuit/channel available) means no suitable circuit or channel was free to complete the call at that moment. It is a lack of resources, not a wrong number. On SIP and E1 trunks it usually means a concurrent channel limit has been reached, yours or the carrier's. Compare active calls with what you contracted and try again.

How Monitor treats cause 34

When the carrier gives the Q.850 cause in the Reason header of the SIP response, Monitor keeps it beside the SIP code on every test call, and it appears in the incident's evidence, ready for the ticket with the carrier. On the English screens, cause 34 reads: no circuit or channel was available.

Like any failure, a test call read as Down is repeated 3 times, every 2 minutes, and only 4 failures in a row open an incident.

What does cause 34 mean in practice?

Q.850 cause 34, No circuit/channel available, means there is no appropriate circuit or channel available to handle the call at that moment. It opens the resource unavailable class: the number may be right and the route may exist, but some leg ran out of capacity. Under RFC 3398 (section 7.2.4.1), cause 34 becomes SIP 503 Service Unavailable, the same response as causes 38, 41, 42 and 47. On E1 with SS7 it arrives in the ISUP REL and, on an ISDN E1, in the DISCONNECT; in SIP, in a Reason: Q.850;cause=34 (RFC 6432), sometimes together with Retry-After. In Asterisk, ${HANGUPCAUSE} is set to 34, the CDR shows hangupcause 34 when it records the field, and DIALSTATUS is usually CONGESTION. In 3CX, the failure appears in the Activity Log with the 503 response received from the trunk.

When it appears

• A SIP trunk with every contracted channel in use.
• An E1 with all 30 channels busy, or with some of its channels blocked.
• Dialer or campaign peaks that exceed the trunk’s limit.
• A carrier interconnection route congested for a specific destination.
• A GSM or E1 gateway with ports unavailable.

How to diagnose it

1. At the time of the failure, run core show channels in Asterisk and compare the total number of calls with the channels in the contract.
2. See whose 503 it is: your SBC’s, your PBX’s or the carrier’s. In sngrep, look at the User-Agent or Server in the response.
3. Check whether the response carries Retry-After and respect the interval before trying again.
4. On an E1, pri show spans and dahdi show status show whether there are blocked channels or alarms.
5. Split by destination: if only one prefix fails, the shortage of channels is at the interconnection, not on your trunk.

What to send the carrier

• Date, time and time zone, ideally with the start and end of the period with failures.
• The A and B numbers and how many attempts failed.
• The Call-ID, the SIP/2.0 503 Service Unavailable line and the Reason with cause=34.
• How many concurrent calls you had at the time and how many the contract provides for.
• The affected destination prefixes.

Frequently asked questions

Does cause 34 mean I need to buy more channels?

Sometimes. If you were using every contracted channel at the time of the failure, then yes, the limit is yours. If there were channels to spare and the cause came from the carrier, the missing circuit is inside its network or at an interconnection, and it is the carrier's to fix. That is why it pays to note how many calls were active when the failure happened.

What is the difference between cause 34 and cause 41?

Both become SIP 503. Cause 34 says specifically that no free circuit or channel was available, in other words capacity. Cause 41 says there was a temporary network failure, unrelated to capacity. In practice, 34 tends to come with traffic peaks, while 41 appears during instability and also as the translation of SIP 500 and 503.