Inbound Call Failover Not Working in SIP Trunking

Symptom

Inbound call failover is expected to automatically route calls to the next available SIP URI when the primary URI is unavailable. However, customers may experience a scenario where calls are failing instead of routing to the next priority SIP URI.

Applies To

• SIP Trunking

Resolution

If inbound calls are not failing over to the next available SIP URI as expected, check whether your SIP server is returning a 4xx response code. These responses may prevent automatic failover because they are generally treated as definitive rejections rather than temporary failures.

To resolve this:

1. Review SIP Logs:

   • Check SIP server logs to determine the response code from your SIP server.
   • If the response is a 4xx error (e.g., 403 Forbidden, 404 Not Found), this may be preventing failover.

2. Update SIP Server Response Handling:

   • Ensure that when a SIP URI is unavailable, the SIP server returns a 5xx response (e.g., 503 Service Unavailable) instead of a 4xx response.
   • A 5xx response signals that the current URI is temporarily unavailable, allowing failover to the next priority URI.

Cause

Failover does not occur when the SIP server returns a 4xx response code because these are treated as permanent rejections rather than temporary failures. Using a 5xx response code allows the system to recognize the issue as temporary and attempt the next available SIP URI.