This could be related to a known issue where secret tokens and headers don't save properly in the Vapi UI. Try using the API instead of the dashboard to create your transfer tool:

bash
curl -X POST https://api.vapi.ai/tool \
     -H "Authorization: Bearer YOUR_API_KEY" \
     -H "Content-Type: application/json" \
     -d '{
      "type": "transferCall",
      "function": {
        "name": "transferCall"
      },
      "destinations": [
        {
          "type": "number",
          "number": "+1234567890",
          "numberE164CheckEnabled": true
        }
      ]
    }'

Sources: - [Discord discussion about API usage](https://discord.com/channels/1211482211119796234/1339709058587820132) - [Known issues with Vapi UI](https://discord.com/channels/1211482211119796234/1338507206051627073) - [Vapi documentation on dynamic call transfers](https://docs.vapi.ai/phone-calling/dynamic-call-transfers)

Could you please add a description to your support.