I am trying to have the agent transfer the call to a outside phone number, but regardless which type of transfer I chose, all transfers fail with the error “An error occurred in this call: call.in-progress.error-transfer-failed” Assistant ID: b7dc43a0-73fb-4374-8c67-d5cc8e0eb7b6 Call ID: e362fea0-b143-45b8-957b-80fad06c556b

Most transfer failures with "call.in-progress.error-transfer-failed" are due to SIP REFER not being enabled on your SIP trunk. To resolve this, please check your SIP provider's settings (e.g., Telnyx) and ensure SIP REFER is enabled for your trunk. If SIP REFER is already enabled, double-check that your

transferCall

tool includes the required

destination

parameter in E.164 format. Example configuration:

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

For more details, see the [Vapi call forwarding guide](https://docs.vapi.ai/call-forwarding). Sources: - [Vapi SIP Trunking Telnyx external number call forwarding troubleshooting](https://discord.com/channels/1211482211119796234/1343963959421829172) - [Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding) - [SIP Trunking](https://docs.vapi.ai/advanced/sip/sip-trunk)

If your transfer destination number is a phone number, it is not a supported transfer type from a web call

I get the same error when using a Twilio number via the Webhook configured with Vapi. The Vapi call log shows this as the last message:

json
{
  "name": "transfer_call_tool",
  "role": "tool_call_result",
  "time": 1750037393359,
  "result": "Success.",
  "toolCallId": "call_...",
  "secondsFromStart": 32.879
}

The Twilio call log does not show any error, nor any transfer related at all, either.

could you please provide a call id?

@mcondie is this helpful?

Checking in, are you still encountering this issue?

Hi, sorry for the late reply. Yes, we are still encountering the issue. Call id with this error is: 2c1e786d-5e27-477a-ab89-dc5d8ad8f688

Nice work! If you need further assistance, please let us know