### Answer I don't have specific troubleshooting information about Elastic SIP trunking call transfer issues with invoice agents in my available sources. To help resolve this, I'd recommend: 1. Check your transfer configuration - Verify that the TransferCallTool is properly configured with the correct destination number for your invoice agent 2. Review call logs - Check your dashboard for any error messages or failed transfer attempts 3. Verify SIP trunk settings - Ensure your SIP provider (Twilio, Telnyx, Vonage, etc.) has proper routing configured for transfers For detailed assistance with this issue, you can: - Visit the [Documentation]() - [Hire a certified Retell partner]() Would you like me to provide general guidance on how call transfers typically work with Retell AI based on my knowledge?

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

### Answer Based on the context that you're using a Twilio phone number via SIP trunking, here are the key configuration steps to verify for call transfers: Verify Twilio SIP Trunk Settings In your Twilio Elastic SIP Trunk, ensure you have enabled call transfer capabilities: [(1) ]() 1. Enable Cold Transfer - This setting must be toggled on in your trunk's general settings 2. Enable PSTN Transfer - This checkbox must be ticked to allow transfers to phone numbers After making these changes, click Save and wait for the confirmation message. Check Termination Configuration Your Termination SIP URI setup is critical for outbound transfers: - Verify you're using a localized termination URI near your region - Ensure there are no spaces in the Termination SIP URI - If using authentication, confirm the username and password are correctly entered in Retell (note: the username is different from the "friendly name") Whitelist Retell's IP Block Your Twilio trunk needs to accept requests from Retell's SIP SBC CIDR block:

18.98.16.120/30

[(1) ]() [(2) ]() Common Transfer Issues According to community reports, transfer failures with Elastic SIP trunking can occur when: [(3) ]() [(4) ]() - The destination number doesn't ring or receive the call [(4) ]() - Configuration settings aren't properly saved [(1) ]() - The phone number has been marked as spam [(4) ]() If transfers continue to f

ail after verifying these settings, check your Twilio logs for specific error messages that can help identify the root cause. [(1) ]()

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Discord message from @michael_18970 on 2025-01-14](https://discord.com/channels/1186424376300679209/1186462578310910112/1328520887544053812) [[4] Transfer Call Fails](https://discord.com/channels/1186424376300679209/1339961704502526002) ​

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