Not being able to transfer a call

Support Help
1

We are experiencing a persistent, repeatable issue with our transfer_call function. The transfer is failing immediately after the agent calls the function, and the error indicates a platform/telephony issue, not an issue with our configuration.

Please investigate the following Call ID, as the error is clearly visible in the detailed logs:

  • Failed Call ID (Latest Attempt): call_851f785e368759db490cc8d684c

  • SIP Error Log: The log entry shows the following specific error: Error transferring call via SIP REFER. SIP status code: 500 SIP error category: telephony_provider_unavailable Error: twirp error unknown: sip status: 500: Server Internal Error

  • Target Transfer Number: The destination number is correctly formatted as E.164 starting with +61

  • Issue: The LLM successfully recognized the intent and executed the transfer_call tool, but the outbound dial failed due to the provider’s internal server error.

We have already verified the destination number is active and that the agent’s number is correctly provisioned.

We require assistance with the SIP 500 error being returned by the telephony provider. Could you please check the detailed Twilio/telephony logs associated with this Call ID?

477 SIP Dial Failed Rate Excessively High
Transfer call issue - complains about E164 formatting when it's valid
500 error on call transfer
2

Based on the SIP 500 error you’re seeing (telephony_provider_unavailable with “Server Internal Error”), this indicates an issue on the telephony provider’s side rather than with your Retell configuration(1).

The disconnection reason telephony_provider_unavailable represents cases where the telephony provider is not available or has errors(1). This is one of the standard error codes that Retell tracks for outbound call issues(1).

Troubleshooting steps:

  1. Check your telephony provider’s logs - Since you’re using custom telephony, you should check with your telephony provider (Twilio, Telnyx, etc.) to see what error occurred on their side(1). The SIP 500 status indicates their server encountered an internal error processing the transfer request.

  2. Verify your SIP trunk configuration - Double-check your configuration and make sure you imported the correct information(1). If the configuration is incorrect, delete the imported number and re-import it(1).

  3. Review detailed logs - The call detailed log contains the disconnection reason, SIP error code, and error message, which should help identify the root cause(1).

  4. Contact your telephony provider - If the configuration appears correct, open a ticket with your telephony provider to investigate the SIP 500 error on their end(1).

For direct support with accessing Retell’s internal telephony logs for this specific Call ID, you can:

The SIP 500 error specifically points to a server-side issue with your telephony provider rather than a Retell platform issue, so working with your provider’s support team will likely be the most direct path to resolution.

**

3

Hello Atticus
I checked your phone number congifration and it seems you have the wrong Termination URI
Termination URI should be the URI from your provider not our URI You can check this Docs it will help you
https://docs.retellai.com/deploy/custom-telephony#custom-telephony-overview
You can also share what provider you are using and I will help you set it up.

4

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”
  1. Tested inside real calls (not just Simulation mode)
  2. 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

Tool Invocation: llamada_soporte

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.”}

Agent ID: agent_1492d0c807bdb78c782b41dcfc

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!

5

Hello,

Please create a new ticket