Hi.. we have been facing this issue for all calls.. as soon as a call is transferred.. the call get's dropped for the customer as well as the agent.. the call transfers are just not working. Example Logs - 7c1f6d3d-47a4-4052-948d-2b925f66251b 4c3ac94b-3623-4e29-8ec0-df0408b50eaf 182f90c1-b4af-4ec9-a898-40fb107650ac Also attaching the Twilio Call Log Screenshot which shows all forwarded calls as just 1 sec or 2 sec and then the call get's dropped. https://cdn.discordapp.com/attachments/1404868309601751193/1404868310364983357/image.png?ex=689cc112&is=689b6f92&hm=bb16d998c069102ae4ee858a88230a0ff47b16730091d3331b4577ed4d117f48& https://cdn.discordapp.com/attachments/1404868309601751193/1404868310998454352/image.png?ex=689cc112&is=689b6f92&hm=49b3f4f2b6a8487dfc12463cfab383ecd8d965d89d583efa5a2396737ddbdbad&

Check if SIP REFER is enabled on your SIP trunk (e.g., Telnyx), as Vapi requires SIP REFER for call transfers—otherwise, calls will drop immediately. Also, ensure your TransferCall tool includes the required

destination

property in its parameters. Here’s a sample configuration:

json
"parameters": {
  "type": "object",
  "properties": {
    "destination": {
      "type": "string",
      "enum": ["+1234567890"], // Replace with your external number in E.164 format
      "description": "The destination to transfer the call to."
    }
  },
  "required": ["destination"]
}

If SIP REFER is not supported or this property is missing, transfers will fail and calls will drop ([Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding)). Source: - [Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding)

I am using Twilio how do I set this up? I don';t think we are using SIP Trunks

Hey, I can help you get the call transfer working with twillio and vapi

great please tell me more

Is your number verified on twillio?

Yes! Verified

that ‘Silence Detected: true’ flag paired with a clean network trace This usually points to an integration-level issue or a misfire in twillio configuration "not" bot or VAPI directly. I’ve fixed similar setups before where the AI just goes dead mid-call when initiating transfer. If you want, DM me...I can walk you through what’s really going wrong here and get it sorted fast.

UPDATE: So it works when I turn on Warm Transfer (Experimental) Everything else is failing and dropping the call.

I'm having the same issue, but we're using Vonage for our numbers - does that also require the use of "sip refer"?

Can you check with Warm Transfer(Experimental) too? That was the only thing that workd in my case.. I did not setup a SIP

Checking if this is resolved/solved for you?

Just tried with "Warm Transfer - Experimental" and regular "Warm Transfer with Message" - with both the call does not get transferred and just ends.

Looking into it