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i am using the transfer call tool named " transfer_call_tool_demo" but my calls are not transfered at all.

details:- SIP Call-Id: cfc8872f-463d-123f-d895-0ec7040a32ad
vapi call id
019ac5a1-b3ce-7444-87b2-033e22baa1d1

I have selected the transfer mode as warm transfer but it is executing as cold transfer (refer screenshot)
https://cdn.discordapp.com/attachments/1443608577066532944/1443608577389498440/Screenshot_2025-11-27_195312.png?ex=694df201&is=694ca081&hm=de463f8e311a2d65db9464905a3e74aa8fef696e9884241349a6a4206387b7f1&

**Your issue occurs because Vapi's warm transfer only works with Twilio; with SIP, the warm transfer message is placed in SIP headers and not voiced out.**  
To address this, contact your SIP provider and ask if they can extract and voice custom SIP headers after a SIP REFER is sent.  
*Example request to your provider:*

```
Can you process custom SIP headers (e.g., X-Warm-Transfer-Message) and play them as audio to the agent when a REFER is received?
```

This may enable the warm transfer experience you expect without switching providers.

Source:
- [Discord discussion on SIP warm transfer limitations](https://discord.com/channels/1211482211119796234/1366650099035541544)

Hi Shubham, thanks for sharing your details. Our team will look into it and get back to you soon with an update.

Hi, Warm transfer only works with Twilio numbers. Since you're using a BYO SIP number for that call ID, the warm transfer feature won't work as expected.