SIP 403 Forbidden on SG BYOC Trunk Re-registration After Carrier IP Rotation

Does anyone know why Genesys Cloud returns a 403 Forbidden instead of a standard 401 when re-registering a BYOC trunk after our Singapore carrier rotates the source IP? The outbound routing fails immediately, and the logs show no clear handshake error.

  • Region: Singapore (ap-southeast-1)
  • Trunk Count: 15 BYOC
  • Error: SIP 403 Forbidden on REGISTER
  • Timestamp: 1400 SGT today

The carrier claims the IP is whitelisted, but Genesys rejects the registration entirely.