### Answer The error "The destination number is not allowed to be called" is occurring because call transfer only works during phone calls, not web calls. [(1) ]() [(2) ]() Your stack trace shows

OrchestratorWebRTC

, which indicates you're attempting a call transfer on a *WebRTC (web call) * session. The documentation explicitly states: > This feature only works during phone calls instead of web calls. It's available for Retell numbers and imported numbers. ## Solution Call transfer functionality is not supported for web calls. To use call transfer, you need to: 1. Use a phone call (PSTN) instead of a web call 2. Ensure you're using either a Retell number or an imported number If you need to transfer calls, you'll need to switch from WebRTC to phone-based calls using Retell's telephony features.

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) ​

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

@AI Evy I am using phone call to do it but it still happens. What can be the reason?

### Answer If you're using a phone call and still seeing "The destination number is not allowed to be called," there are several potential causes: 1. Custom Telephony Configuration Issues If you're using custom telephony via SIP trunking, the transfer relies on your telephony provider to support SIP REFER protocol and proper routing. [(1) ]() [(2) ]() Check your telephony provider's settings for: - Termination settings for outbound calls [(1) ]() - SIP REFER configuration (for cold transfers) [(2) ]() - Transfer to PSTN settings 2. Incorrect Termination URI A common issue is having the wrong Termination URI configured. The Termination URI should be the URI from your telephony provider, not Retell's URI. [(3) ]() If the configuration is incorrect, delete the imported number and re-import it. [(1) ]() 3. Destination Number Format Ensure the transfer destination number is in correct E.164 format (e.g.,

+14157774444

) . [(4) ]() [(5) ]() 4. Telephony Provider Restrictions Your telephony provider may have restrictions on which numbers can be called for transfers. Some providers require specific configuration to allow SIP REFER and PSTN transfer. [(2) ]() Debugging Steps: 1. Check the call detailed log in your dashboard—it contains the disconnection reason, SIP

error code, and error message. [(6) ]() 2. Check the logs in your telephony provider to see where the call routing is failing. [(1) ]() 3. Verify your SIP trunk authentication is set up properly. If the issue persists, you may need to open a ticket with your telephony provider to investigate their call routing configuration. [(1) ]() [(3) ]()

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Agent transfers the calls to itself](https://community.retellai.com/t/agent-transfers-the-calls-to-itself/699) [[3] Not being able to transfer a call](https://community.retellai.com/t/not-being-able-to-transfer-a-call/225) [[4] Register Phone Call](https://docs.retellai.com/api-references/register-phone-call) [[5] SIP Trunk Outbound Call - No SIP INVITE reaching my server](https://community.retellai.com/t/sip-trunk-outbound-call-no-sip-invite-reaching-my-server/760) [[6] Debug outbound connection issues](https://docs.retellai.com/reliability/debug-outbound-call) ​

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

### Answer The "Transfer destination is not supported" error typically indicates one of these issues: 1. Using Dial-to-SIP Method Instead of Elastic SIP Trunking If you're using the Dial-to-SIP URI method for custom telephony, you will not be able to use Retell's transfer call feature. [(1) ]() The documentation states: "When using this method, you will not be able to utilize Retell's transfer call feature, as we do not have access and control over the telephony provider and number, and cannot initiate transfer for you." 2. Retell Number Calling Restrictions If you're using numbers purchased directly from Retell, they currently can only make calls to US numbers. [(2) ]() Recommended Solutions: - If you need call transfer functionality, use Elastic SIP Trunking instead of the Dial-to-SIP method—this is the recommended approach and supports all telephony features including transfers. [(1) ]() - If using custom telephony with Dial-to-SIP, you can implement your own transfer logic using a custom function to trigger the transfer through your telephony provider. - Verify your transfer destination number is in valid E.164 format and is a supported destination for your telephony setup. Could you confirm which telephony method you're using—Elastic SIP Trunking, Dial-to-SIP, or Retell-purchased numbers?

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Debug outbound connection issues](https://docs.retellai.com/reliability/debug-outbound-call) ​

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

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