I have setup an agent for a client using a twilio number (I connected it via SIP trunking), I am using the call transfer function in retell.
The issue I am facing is the call transfer is going through to the correct destination but the ring only lasts for 2 rings. If the person does not answer within that time frame the call transfer fails. The timeout in the call transfer function is set to 50 seconds.
I did speak to the twilio team and they said the SIP trunk is set up correctly.
Based on the available documentation, the timeout you’re referencing (50 seconds) appears to be the Agent detection timeout for warm transfers, not the ring duration.(1)(2)
For warm transfers specifically, the Agent detection timeout is “the maximum amount of time the AI agent will wait to determine whether the transfer target is a human. The caller is connected only if human detection succeeds within this timeframe. Otherwise, the transfer is marked as failed. The default timeout is 30 seconds.”(1)(2)
If you’re experiencing only 2 rings before the transfer fails, this suggests the issue may be related to:
Transfer type configuration - Verify whether you’re using cold transfer or warm transfer. Cold transfer simply transfers the call to a destination number, while warm transfer includes human detection and additional settings.(1)(2)
Human detection settings - If you’re using warm transfer with “Enable human detection” turned on, the agent checks if a human is present after the transfer target answers, and the original caller is only connected once a human is detected.(1)(2)
Auto-greet setting - When enabled, “the agent will immediately say ‘Hello’ when the transfer target picks up. This encourages a response, increasing the likelihood of detecting a human.”(1)(2)
Since Twilio confirmed your SIP trunk is configured correctly, I recommend checking your transfer node configuration in Retell AI to ensure the transfer type and human detection settings align with your requirements.
For additional support with this specific configuration issue, you can:
Thank you for your response. However, I need to clarify - my configuration is already set up exactly as described in your documentation. The issue is that the system is NOT respecting the configured timeout value.
My Current Configuration (See Attached Screenshots):
The Problem:
Despite the Detection Timeout being set to 50 seconds, the system is sending a SIP CANCEL after only ~10-12 seconds (2 rings). This is NOT the configured behavior.
According to your documentation: “The maximum amount of time the AI agent will wait to determine whether the transfer target is a human… default timeout is 30 seconds.”
My timeout is set to 50 seconds, yet the call cancels after 10-12 seconds. This is 40 seconds shorter than configured.
Twilio has confirmed:
The destination phone IS ringing
The CANCEL is coming from Retell AI’s platform
This happens after only 2 rings, not 50 seconds
This appears to be a platform bug, not a configuration issue.
Can you please escalate this to your engineering team for investigation? I’m happy to provide specific call IDs, timestamps, or any other data needed.
Attaching screenshots of my configuration for reference.
Thank you for providing those call IDs. To help investigate your specific call transfer issues, the Retell AI support team will need to review the detailed call logs associated with those IDs.(6)
Based on similar cases in the community, when call transfers fail or timeout prematurely with custom SIP trunking through Twilio, the issue often relates to:
Telephony Provider Configuration Issues
Your SIP trunk or telephony provider may have incorrect routing rules that cause premature call termination.(4) For warm transfers specifically, the transfer uses SIP DIAL and sets from and P-Asserted-Identity headers.(1)(3)
Warm Transfer Permissions
Warm transfer requires different permissions from Twilio compared to cold transfer.(9) As noted in community discussions, “warm transfer is different behavior that will require more permissions from Twilio.”(9)
Debugging Steps
Check the logs in your telephony provider to see where the call routing is failing.(4) The SIP connection log is available in the call logs and is useful for diagnosing the failure reason.(2)
Since you’ve confirmed with Twilio that your SIP trunk is set up correctly, I recommend:
Sharing these call IDs with Retell AI support directly so they can review the detailed logs
Asking Twilio specifically: “What permissions do I need to enable warm transfer for any number?” and showing them the error from your call logs(9)
