Issue with transfer_call — “Transfer destination is not supported” even after configuration fixes

General
1

Hi everyone,

I’m running into a persistent issue with the transfer_call function and I’m hoping someone can help clarify what I’m missing.

I have an agent configured with the transfer_call tool. The tool is enabled, correctly named (transfer_call), and points to a valid phone number in E.164 format.

However, every time the agent tries to transfer, I get this response:

Transfer destination is not supported.. Please inform the customer that the transfer did not go through and offer to try again or assist them directly.

Here’s what I’ve already tried:

  1. Confirmed the tool name matches exactly (transfer_call)

  2. Verified the agent has the function enabled in Functions

  3. Configured a static destination number (first mobile, then a landline)

  4. Set number format to E.164

  5. Tried both Warm and Cold transfer modes

  6. Disabled:

    • “Transfer only if a human answers”

    • tool_call_id: dff12027e706e72b

      {
  "execution_message": "Vale, Sergio, como es sobre información de una lavadora, esto lo ve mejor el departamento comercial. Te paso ahora mismo con ellos para que puedas hablar con Viviana o con quien te pueda ayudar del equipo comercial."
}

      Tool Result

      tool_call_id: dff12027e706e72b

      {“status”:“Transfer destination is not supported.. Please inform the customer that the transfer did not go through and offer to try again or assist them directly.”}

  7. Tested inside real calls (not just Simulation mode)

  8. Confirmed other agents in the same workspace are able to transfer successfully using their own transfer tools

Despite all this, this particular agent always returns:

Transfer destination is not supported

What I find strange:

  • Another agent in the same account can successfully transfer to a different mobile number

  • The tool appears correctly in the Functions list

  • The logs show the tool is being invoked correctly, but it fails instantly

Questions:

  1. Is there any additional permission or outbound routing setting tied specifically to transfer destinations?

  2. Does each agent or each phone number need to be individually approved for outbound transfers?

  3. Is there anything else that could cause “Transfer destination is not supported” even when the configuration looks correct?

Any guidance, examples, or screenshots of a working configuration would be greatly appreciated.

Thanks in advance!