SIP INVITE 403 Forbidden during BYOC failover in Asia/Singapore

Why does the /v2/voice/trunks endpoint returns a 403 Forbidden error specifically when traffic fails over to secondary BYOC trunks in the Asia/Singapore region? The primary trunks function correctly, but the secondary ones reject SIP INVITE requests immediately after the failover trigger.

Steps to reproduce:

  1. Configure two BYOC trunks for the Asia/Singapore region with identical SIP credentials.
  2. Set up an outbound routing policy with the primary trunk first and the secondary trunk as failover.
  3. Simulate a primary trunk outage by disabling the SIP registration.
  4. Initiate an outbound call through the Architect flow.
  5. Observe the 403 error in the call logs and the /v2/voice/trunks API response.

The error message indicates ‘Invalid SIP credentials,’ but the credentials are verified correct. This issue occurs consistently across all 15 trunks in the region. Is there a known limitation with BYOC failover logic in this region? Any insights into the underlying cause would be appreciated.