The remote SIP server temporarily refuses to process calls.
Likely causes
- Carrier maintenance
- Overload
- Channel limit reached
- Route or SBC unavailable
Checks in priority order
- Read Retry-After
- Check trunk status
- Compare inbound and outbound calls
- Use the backup route if designed
When to escalate
Escalate when the failure affects multiple users, a production dependency is unavailable, or logs show a component outside your control. Include timestamps, scope, tests already performed, and the last known working state.