I have an assistant that receives calls via a SIP TRUNK phone number. Now, I'm setting it up to transfer calls using a SIP URI. The transfer is working, but the warm transfer isn't—VAPI is sending a blind transfer instead. Here’s my transfer call tool:

{
  "type": "transferCall",
  "async": false,
  "function": {
    "name": "transferCallDynamic",
    "parameters": {
      "type": "object",
      "required": [
        "destination"
      ],
      "properties": {
        "destination": {
          "enum": [
            "sip:200@mydomain.sip.twilio.com",
            "+5511998338705"
          ],
          "type": "string",
          "description": "The destination to transfer the call to."
        }
      }
    },
    "description": "..."
  },
  "destinations": [
    {
      "type": "sip",
      "sipUri": "sip:200@mydomain.sip.twilio.com",
      "transferPlan": {
        "mode": "warm-transfer-say-summary",
        "sipVerb": "refer",
        "summaryPlan": {
          "enabled": true,
          "messages": [
            {
              "role": "system",
              "content": "You are an expert note-taker. You will be given a transcript of a call. Summarize the call in 2-3 sentences. DO NOT return anything except the summary."
            },
            {
              "role": "user",
              "content": "Here is the transcript:\n\n{{transcript}}\n\n"
            }
          ]
        }
      }
    },
    {
      "type": "number",
      "number": "+5511998338705"
    }
  ]
}

Could you support me to fix it?

Hi there! 👋 I see you're having an issue with warm transfers using a SIP URI, where VAPI is performing a blind transfer instead of the expected warm transfer with summary. Based on the information I have, I can help troubleshoot this issue. Let me share some potential causes and solutions: ### SIP REFER Handling One of the most common issues with warm transfers is related to SIP REFER handling. According to [troubleshooting information](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1333747605628391434), VAPI uses SIP REFER messages for call transfers, and the receiving system must be properly configured to handle these messages. Make sure that your Twilio SIP domain (

mydomain.sip.twilio.com

) is configured to properly handle SIP REFER messages. If the receiving system doesn't support SIP REFER, the transfer will likely fail or result in a blind transfer. ### Warm Transfer Timing Another potential issue could be related to timing. According to [implementation details](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1316539646808686602), a common problem is that the warm message is not played before the call transfer. The call flow logic might need to be adjusted to wait for the message playback to complete before initiating the transfer. ### SIP Verb Alternatives You're currently using

"sipVerb": "refer"

in your configuration. According to [troubleshooting information](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1343963959421829172), some users have experienced issues with SIP REFER, and their provider suggested eliminating SIP REFER. You might want to try using a different SIP verb if your provider supports it. ### Check Your Transfer Mode

Hey! To help track down this issue, could you share: - The call ID This would really help us figure out what went wrong!

CallId's 5d60cf16-01bd-4e8a-965f-28152ebf56d7 f9a7004d-2ed2-445c-b718-de31758ecb32 056f8e07-5e0f-44a9-818f-ba263f3e85c6

@Seygi can you try another call with

warm-transfer-wait-for-operator-to-speak-first-and-then-say-summary

?

right now

@Seygi I am checking with the team about this, and I will get back to you with a proper answer.

@Shubham Bajaj tks

Hey Seygi, I checked with the team, the warm transfer with the summary part only works for now Twilio numbers. As you are using a number which is VAPI provided under free telephony, this feature won't work for it. That's why the transfer was successful, but eventually, the summary wasn't voiced out.

Im not using a number under free telephony Im using a Twilio sip trunk with a Twilio phone number Is it possible to use the warm transfer for this setup?

Your phone number type is BYOP not twilio that's why during the warm transfer only call was forwarded. Shared an screenshot with you in DM kindly check.