Based on the available sources, a 403 error in Retell AI typically indicates a telephony_provider_permission_denied error(1). This represents cases where the SIP trunk authentication failed(1).
What Causes a 403 Error
When a call has a disconnection_reason of telephony_provider_permission_denied, this indicates the telephony provider is denying permission for the operation(1). 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).
Troubleshooting Steps
Check Call Detailed Logs: Review the detailed call logs for the specific SIP error code and error message(1). This should provide more context about why the permission was denied.
Verify Phone Number Configuration: If you’re using numbers 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).
SIP Configuration for Transfers: 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(1).
If the configuration doesn’t resolve the issue and you’ve confirmed the destination is valid, you should contact Retell support directly for account-level investigation(1).
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