### Answer The "Transfer call cannot be completed, probably don't have the permission to perform the transfer" error typically stems from telephony provider configuration issues. [(1) ]() Common causes and fixes: 1. SIP trunk settings – If you're using a SIP-trunked number, verify your trunking configuration is correct. Check that your termination URI is unique and credentials/IP ACLs are properly set up. 2. Transfer destination format – Ensure the transfer number is in e.164 format (e.g.,

+15551234567

) or a valid SIP URI (

sip:username@domain

) . [(2) ]() 3. Outbound transport method – Try changing the outbound transport method to either TCP or UDP. [(1) ]() 4. Provider permissions – Your telephony provider may not allow call transfers. [(3) ]() One user resolved this by checking their specific provider settings. To troubleshoot: - Check the call history and detailed log for SIP error codes [(4) ]() - If using custom telephony, double-check your configuration and verify credentials were imported correctly - Contact your telephony provider to check for errors on their side Could you share your call ID and whether you're using a Retell number or custom telephony (SIP trunk) ?

​ To learn more, see the sources I considered: [[1] Transfer Call Permission Error - Need Configuration Assistance](https://discord.com/channels/1186424376300679209/1450235811470770359) [[2] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[3] Permissions to transfer call](https://discord.com/channels/1186424376300679209/1390046901230899210) [[4] 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 Since you're using the transfer call function directly through Retell, here are key things to check: 1. Verify the transfer destination format The transfer number must be in e.164 format (e.g.,

+12137771234

) or a valid SIP URI (e.g.,

sip:user@retellai.com

) . [(1) ]() 2. Check your call type Call transfer is only supported for phone calls, not web calls. [(2) ]() 3. Review your call logs Check the SIP connection log in your call logs—call transfer failures occur for similar reasons as outbound call failures, and the SIP logs can help diagnose the specific failure reason. 4. Possible telephony provider issue In similar cases, users found the error originated from their telephony provider. One user with Telnyx received an "Unverified origination number" error (D51) from their provider's side. [(3) ]() Next steps: - Share your call ID so support can check the detailed logs - Confirm whether you're using a Retell-purchased number or an imported number (via SIP trunk from Twilio, Telnyx, etc.) - If using custom telephony, verify your termination URI and credentials are correctly configured Can you provide the call ID and let us know which telephony setup you're using?

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Debug call transfer failure](https://docs.retellai.com/reliability/call-performance) [[3] Transfer Call Permission Error - Need Configuration Assistance](https://discord.com/channels/1186424376300679209/1450235811470770359) ​

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

### Answer For viewing call logs, you have two places to check: Retell Dashboard: Your call recordings, transcripts, and detailed logs (including

public_log_url

) are available after the call ends through the Retell dashboard or API response. [(1) ]() The

public_log_url

contains details about all requests and responses received in the LLM WebSocket, plus latency tracking for each turn-taking—helpful for debugging and tracing. Twilio Console: To view Twilio telephony logs, navigate to the Twilio Console → Monitor, and search for specific call logs using the Call SID. [(2) ]() You can log the Call SID from each transfer attempt in your Retell tool or webhook so you can trace it later. Important checks for your transfer issue: Since you're using Twilio but transferring to a number not associated with Twilio: - Check whether the transfer attempt is logged at all in Twilio - If it's missing, review your SIP trunk and transfer setup (call forwarding, REFER permissions, etc.) - If it is logged, inspect the SIP response code for clues like "Transfer not allowed" or "Busy" Also verify your Twilio Elastic SIP Trunk has the correct Geographic Permissions enabled for the country you're trying to transfer to—search "geo" in Twilio to find "Voice Geographic Permissions" and select the countries you need to dial. [(1) ]()

​ To learn more, see the sources I considered: [[1] Register Phone Call](https://docs.retellai.com/api-references/register-phone-call) [[2] Twilio telephony logs](https://discord.com/channels/1186424376300679209/1361768314879213790) ​

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

### Answer The Twilio Error 32201 indicates your ACL needs to include Retell's IP addresses. Here are the IP blocks to whitelist: Retell SIP Server IP Blocks: [(1) ]() -

18.98.16.120/30

(All regions) -

143.223.88.0/21

(United States only) -

161.115.160.0/19

(United States only) Add all three IP ranges to your Twilio Elastic SIP Trunk's ACL configuration. Note: Some non-US traffic might come from IPs not listed above, but will be migrated to the first IP block by 12/03/2025. Alternative approach: If you've set up credential-based authentication (username/password) , you don't need the IP ACLs—only one or the other is required. [(2) ]() One user resolved a similar "Telephony Permission Denied" error by adding the

143.223.88.0/21

and

161.115.160.0/19

ranges. To configure in Twilio: 1. Go to your Elastic SIP Trunk → Termination settings 2. Add the IP ranges above to your ACL, or 3. Use credential-based authentication instead (specify username/password when importing the number to Retell) [(1) ]()

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] "Telephony Permission Denied" (Twilio)](https://discord.com/channels/1186424376300679209/1446239879112556625) ​

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