Hi VAPI team! Urgent issue with call transfers using Telnyx SIP trunk. Problem: Outbound call transfer fails — VAPI shows "Transfer Completed Successfully" but immediately drops with "Call ended due to transfer failure" Telnyx confirmed: SIP REFER is accepted on their end and forwarded to VAPI IP 44.238.177.138 but VAPI server returns 503 Service Unavailable — this is a VAPI-side bug. Call ID: 019d49b4-82b3-7ffa-90f2-287c34383369 Org ID: c08e0686-1b2b-49c4-8a89-db03539cc5ea Please help! 🙏

Action Required: Your issue matches the escalation criteria in the Vapi documentation. Please open a support ticket and include the following details:

- Call ID: 019d49b4-82b3-7ffa-90f2-287c34383369
- Org ID: c08e0686-1b2b-49c4-8a89-db03539cc5ea
- Timestamp and provider logs showing SIP REFER acceptance
- Description of the 503 error from Vapi

This will help the Vapi team investigate and resolve the transfer failure promptly. Reference: [Vapi Debug Call Forwarding Drops](https://docs.vapi.ai/phone-calling/in-call-control/transfer-calls/debug-forwarding-drops) Source: - [Vapi Debug Call Forwarding Drops](https://docs.vapi.ai/phone-calling/in-call-control/transfer-calls/debug-forwarding-drops)

Hi, Thank you for your patience while we investigated the issue. We see Telnyx returned a

403 Forbidden

error when Vapi attempted to transfer the call. This indicates that the Telnyx account or connection currently does not have the required permissions to execute call transfers through their Call Control API. To help resolve this, please log in to your Telnyx portal at

portal.telnyx.com

and review the following: • Check that it's set up as a call control connection, not a standard sip connection. standard sip connections can make/receive calls but can't do transfers through the api • If it's already a call control connection, go to auth → api keys and make sure the api key you gave vapi has call control write permissions enabled • Also check that call transfer isn't restricted on your telnyx account level as some accounts need this feature explicitly enabled

Hey @Saeed , I can help you debug this quickly. I’ve worked with VAPI + Telnyx SIP setups before this kind of 503 on REFER usually points to how VAPI is handling the transfer target or SIP headers. If you’re open, I can: Check your transfer config + SIP flow Review headers / routing issue causing the 503 Suggest a working fix or workaround Shoot me a DM and let’s fix it.