Realtime API SIP inbound calls failing again before webhook dispatch

From Twilio:

Thank you for your response and for providing detailed information and sharing the additional context. That help clarify this issue.

I understand now that this isn’t isolated to the xxxx trunk. Since all of your OpenAI-related trunks started failing at roughly the same time with the same 400 Bad Request behavior, and nothing changed on your side, that points away from a simple trunk configuration issue.

The fact that your fallback setup using Twilio Media Streams + OpenAI Realtime WebSocket API is working. Since that flow uses the same Twilio account, phone numbers, and OpenAI projects, it does suggest the issue is specifically tied to traffic going through sip.api.openai.com.

From the call samples we reviewed, we can confirm that Twilio is attempting to send the outbound SIP INVITEs to the OpenAI SIP endpoint and that the calls are failing immediately with a 400 Bad Request response from the far end. At this point, we’re not seeing evidence that Twilio is blocking, rate-limiting, or intentionally rejecting the traffic.

Please find the SIP ladder below showing that Twilio is successfully sending the SIP INVITE to the OpenAI SIP endpoint and receiving a 400 Bad Request response back from the OpenAI server during call setup.

We also appreciate you sharing the OpenAI community thread and the OpenAI support case reference. Seeing similar reports from multiple developers, providers, and regions around the same timeframe is useful context and does make this look more consistent with an upstream SIP ingress issue.

For now, based on the traces reviewed so far:

• The SIP INVITEs are leaving Twilio successfully

• The 400 Bad Request appears to be coming back from sip.api.openai.com

• We do not currently see signs of Twilio-side filtering or throttling affecting these calls

Please feel free to reach out if you have any additional questions or need further clarification on this issue. I’ll move this case to ‘Pending’ while I await your response. If I don’t hear back from you, the case will close. You’ll also receive reminders along the way.

Kind regards, Twilio Support