I am attempting to perform a call transfer using the controlUrl, but the original call gets dropped when transferring to an external number. Context Incoming calls arrive through my Bandwidth SIP trunk. The inbound call is successfully received by Vapi. I then attempt to transfer the call to a PSTN number outside the trunk. In the Vapi logs, the transfer appears successful and the call ends with: endedReason: "assistant-forwarded-call" However, the original caller is disconnected instead of being transferred. Expected Behavior The inbound caller should be blind transferred to +18004377950. Actual Behavior The call terminates instead of transferring. Vapi reports the transfer as successful. Call Details id: 019cbe70-6765-733b-ba27-32ea26fe2461 timestamp: 2026-03-05T14:39:23.493Z I am using the controlUrl and sending the following payload: { "type": "transfer", "destination": { "type": "number", "number": "+18004377950", "callerId": "+15128060410", "transferPlan": { "mode": "blind-transfer", "sipVerb": "dial" } }, "content": "Transferring you to the destination." }

Actionable Next Step: Since Vapi reports

endedReason: "assistant-forwarded-call"

but the call drops, the issue is likely with your SIP provider (Bandwidth). Check your provider's call logs for transfer errors: 1. Locate the

phoneCallProviderId

from your Vapi call object. 2. Search for this call in your Bandwidth portal and review the call timeline for transfer-related errors or rejections. Tip: If possible, capture SIP packets and look for REFER or 4xx/5xx error responses to pinpoint the failure. For more details, see the [Vapi debug guide](https://docs.vapi.ai/phone-calling/in-call-control/transfer-calls/debug-forwarding-drops). Source: - [Debug call forwarding drops](https://docs.vapi.ai/phone-calling/in-call-control/transfer-calls/debug-forwarding-drops)

Hey, I see why the call drops instead of transferring, this usually happens when the SIP trunk or transfer method doesn’t support outbound PSTN blind transfers directly. I can help troubleshoot and walk you through a reliable approach so the caller stays connected. Quick question: is your Bandwidth trunk set up to allow outbound calls to external numbers from Vapi transfers? @larry

@James Christopher Yes I believe so. If I call one of the phone numbers that vapi provides and initiate a transfer to the same number, it works fine. Does this answer your question ?

That’s helpful @larry if the transfer works when dialing another Vapi number, it means your transfer logic and payload are likely set up correctly. The issue is probably happening at the PSTN routing layer between Vapi and your Bandwidth trunk, where outbound transfers to external numbers sometimes require specific trunk permissions, caller ID formatting, or SIP handling. I’ve seen a few similar cases with Bandwidth SIP + Vapi, and it usually ends up being a small configuration detail causing the call to drop instead of bridging. If you’re open to it, feel free to message me privately and I can take a closer look at the setup with you and help pinpoint exactly where it’s breaking. Should be a pretty quick fix once we check the signaling. 👍 @larry