Hey i have a problem, We’re currently trying to fix a call transfer issue in Vapi for a voice assistant. It is supposed to transfer callers to an internal phone number using Twilio, but the transfer starts and then fails instead of reaching the destination phone properly. At this point, we’ve already ruled out a few obvious causes. The target number is being sent in the correct international format, and we also changed the transfer setup so it no longer relies on the earlier AI summary step that was causing trouble before. So that part is no longer the main issue. On the Twilio side, it looks like Twilio is actually trying to place the outbound call. In the call details, that leg even shows up as completed, so this does not look like a simple invalid number or basic permissions problem. The strange part is that the destination mobile phone still does not ring normally, while Vapi continues to report the overall transfer as failed. So our current understanding is this: the transfer is being triggered correctly, but the actual handoff is not completing properly. The problem now seems more likely to be somewhere between Vapi and Twilio during the transfer itself, or on the destination side, for example voicemail, call forwarding, call screening, or some other network behavior that prevents the call from arriving like a normal incoming call.

Hi, when your destination mobile receives a call from a twilio number it doesn't recognise, a few things can silently intercept it before it rings: • the phone's built-in spam/fraud screening (common on ios and android) auto-declines calls from unknown numbers • carrier-level spam filtering — twilio numbers without verified caller id often get flagged • voicemail picking up immediately (set to 0 rings or already on another call) • call forwarding or do-not-disturb on the destination phone • the number being on a carrier blocklist how to fix it here are the steps in order of most likely to solve it: 1. check your twilio call logs for the failed leg • go to twilio console → monitor → logs → calls • find the outbound call to the destination number at the time of the failed transfer • look at the

ChildCallSid

— this is the leg twilio placed to the destination • check its

CallStatus

and

AnsweredBy

fields. if it shows

no-answer

or

machine_start

in under 1 second, that confirms voicemail or screening intercepted it 2. set a verified caller id on the transfer in your vapi transfer config, set the

callerId

so the destination sees a recognisable number instead of a random twilio number. this reduces spam screening rejections: 2.

{
  "type": "number",
  "number": "+1XXXXXXXXXX",
  "transferPlan": {
    "mode": "blind-transfer",
    "callerId": "+1YOUR_VERIFIED_NUMBER"
  }
}

2. the

callerId

should be a number the destination phone recognises — ideally your business number. it needs to be verified in your twilio account first.