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SIP REFER Requests from Vapi aren’t being handled correctly. 

Does Vapi’s device or software support REFER and handle NOTIFY messages? 

Is the target device (192.76.120.10) capable of receiving REFER and completing transfer.

<@1332314970821754941> Are you trying to perform a SIP REFER on a BYO SIP Trunk? or via the VAPI connected Twilio/Telnyx numbers ?

<@1102203628346953868> VAPI connected to Telnyx

This is a major issue we are having as well. Waiting on support resolve issue.

You need to enable the REFER permission from the Telynx team, and then send the SIP refer like this: sip:your-transfer-destination@sip.telynx.com.

SIP REFER has been enabled. Where exactly do I send the SIP destination under?

<@1297971182385102892> Is the issue fixed your end?

There’s nothing on our end to fix. Vapi needs to process the INVITE properly like any other SIP endpoint and not manipulate the realm.

We can’t be the only ones experiencing this issue…

We are not manipulating any Realm here. In case you your confident, you can present the document to prove your point, I'll review it. If it's an issue on our side, we'll definitely fix it.

<@1332314970821754941> For Vapi-Telnyx integration, SIP destinations need to be configured in the transfer tool destinations or assistant settings using the specific Telnyx SIP format: sip:your-transfer-destination@sip.telnyx.com

Configure your transfer destination in the Transfer Call Tool:
```json
{
  "type": "transferCall",
  "destinations": [
    {
      "type": "sip",
      "sipUri": "sip:your-transfer-destination@sip.telnyx.com",
      "description": "Transfer to your endpoint",
      "transferPlan": {
        "mode": "blind-transfer",  // or other transfer modes
        "sipVerb": "refer"  // Uses SIP REFER (default)
      },
      "sipHeaders": {
        // Optional custom SIP headers
        "X-Custom-Header": "value"
      }
    }
  ]
}
```

`notes`
URI Format: Must be sip:destination@sip.telnyx.com
Domain: Always use sip.telnyx.com as the domain for Telnyx routing
Destination: Replace your-transfer-destination with your actual SIP endpoint identifier


Let me know if you require further help!!

I provided it to Sagar today in a separate email. Please get with him. Thanks

Telnyx phone number’s connected for inbound calls. This is my configuration for call transfer for an external number.
https://cdn.discordapp.com/attachments/1378742860253499402/1379758007188983919/Screenshot_2025-06-04_at_10.38.04.jpg?ex=68416747&is=684015c7&hm=4d624fe149bab13efbc26f5207ad5d36950ffdbe280967cf2fde5b317aff6bd8&