This call is from a number purchased through the Retell platform. I’m unable to troubleshoot on my end with Twilio.
I have confirmed via outbound test calls that the system is using the standard INVITE protocol and is correctly using the Retell agent number as the caller ID. There are no custom headers configured.
Could you please investigate this on your end to see why the telephony provider is denying the transfer permission for this Org?
I’ll help you troubleshoot the telephony_provider_permission_denied error you’re experiencing with call transfers.
When a call has disconnection_reason of telephony_provider_permission_denied, this represents cases where the SIP trunk authentication failed(1).
For numbers purchased through Retell, here are the key troubleshooting steps:
Check Call History and Detailed Logs
The call detailed log contains the disconnection reason, the SIP error code, and the error message(1). In most cases, this provides enough information to identify the root cause(1).
For Retell-Purchased Numbers
Since you’re using a number purchased from Retell, verify that the destination number can accept the call(1). Currently, numbers purchased from Retell can only make calls to US numbers(1).
Call Transfer Configuration
For call transfers to work properly with SIP trunking, your configuration must allow SIP REFER and PSTN transfer, with the transferee’s number showing as caller ID(2).
Next Steps
If the configuration doesn’t resolve the issue and you’ve confirmed the destination is a valid US number, you should:
Review the detailed call logs for the specific SIP error code
Contact Retell support directly for account-level investigation