### Answer The

403 Forbidden

error with

telephony_provider_permission_denied

indicates that the SIP trunk authentication failed or the telephony provider is denying permission for the transfer. [(1) ]() Here are some things to check: 1. Caller ID Configuration For call transfers, you can configure which caller ID shows up to the transfer destination. [(2) ]() If you're trying to show the user's number on the transfer, note that the telephony provider must support caller ID override for this feature to work. If caller ID override is not supported, the transfer would fail. 2. Transfer Type Considerations - For warm transfer, it uses SIP DIAL with

from

and

P-Asserted-Identity

headers set to the user's number. - For cold transfer, it uses SIP REFER, and it's up to the telephony provider to support caller ID override. - Retell Twilio numbers support showing user's number on both warm and cold transfer, while Retell Telnyx numbers only support this when using SIP REFER via cold transfer. *3. If Using Custom Telephony (SIP Trunk) * Your configuration must allow SIP REFER and PSTN transfer for call transfers to work properly. [(3) ]() Recommended Next Steps: 1. Try switching the caller ID setting to show the Retell Agent's number instead of the user's number to see if that resolves the permission issue. [(2) ]() 2. If using custom telephony, verify your SIP trunk permissions allow transfers. 3. Check the detailed call logs for more specific SIP error information. [(1) ]() 4. If the issu

e persists with a Retell-purchased number, contact Retell support with your call ID for account-level investigation.

​ To learn more, see the sources I considered: [[1] Debug call disconnection](https://docs.retellai.com/reliability/debug-call-disconnect) [[2] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[3] 403 Forbidden Error on Transfer - Org ID org_HgizBlvxUIw0ezxb](https://community.retellai.com/t/403-forbidden-error-on-transfer-org-id-org-hgizblvxuiw0ezxb/717) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.