Vapi is the Voice AI platform for developers.

Vapi AI

Occasionally when Vapi calls the built in transfer_call tool configured to a destination of a real phone number using the +1 prefix it doesn't actually transfer the call. It just ends the call instead.

And call ID where it did that is 019e8de6-5988-7000-8331-bf48e1a8ac11

The destination phone number should've receieved a call to accept the forwarding but it never receieved it.

Hi,
Thanks for the call ID. We've investigated `019e8de6` and found what happened.

Your transfer configuration is correct, and the `+1` prefix was properly handled. The issue is with how blind transfers work: Vapi sends a SIP REFER to Twilio and immediately considers the transfer "done," ending its own call leg. But if Twilio fails to actually connect to the destination (carrier routing issue, network timeout, etc.), that failure happens silently after Vapi has already disconnected. That's why the destination never rang but the call log shows "assistant-forwarded-call."

**How to reduce this:**
Switch your transfer tool to **attended transfer** mode instead of blind transfer. With attended transfer, Vapi keeps its call leg active until the destination actually picks up. If the connection fails, your assistant can handle it gracefully (retry, take a message, etc.) instead of the call just dropping.