When calls fail before answer, the SIP response tells you which side to chase. The common ones on interconnects, translated:
The ones that are usually the destination
- 404 Not Found / 484 Address Incomplete: the number is unallocated or the digits are wrong. Check the dialled format (full E.164, no local prefixes).
- 480 Temporarily Unavailable / 408 Timeout: the far end did not respond - handset off, network unreachable, or paging failed. Retry behaviour is normal here.
- 486 Busy Here: genuinely busy. Not a route fault.
The ones that are usually policy or commercial
- 403 Forbidden: the call was refused - commonly an unauthorised IP, a destination not open on your account, or a blocked prefix (fraud controls). If you get 403 on traffic that worked yesterday, check whether the destination tripped a fraud rule before assuming an outage - see "Fraud controls on wholesale traffic".
- 503 Service Unavailable: congestion or a capacity cap - either your CPS/session limit with us, or downstream congestion. Sustained 503s on one destination are reroute requests waiting to happen; tell us.
The ones that need both sides
- 500/502 series: something broke mid-path. Grab the timestamp and example and send it over - these are ours to chase downstream.
Reporting failed-call issues
Include: dialled number, timestamp with timezone, the SIP response you received, and the volume affected (one number? a prefix? a whole destination?). "Destination X returning 403 since 09:20 UTC, 3 examples attached" gets a same-shift answer; "calls failing" does not.
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