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?

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.

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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.