### Answer This error typically indicates an issue with your telephony provider rather than a Retell-wide outage. [(1) ]() Based on similar reports, here are the common causes: 1. Telephony provider issues — Call transfer failures often occur for similar reasons as outbound call failures. The SIP connection log in your call logs can help diagnose the specific failure reason. 2. Provider-specific errors — In one case with Telnyx, the error was traced to "Unverified origination number D51" — a provider-side issue that required contacting Telnyx support. [(2) ]() 3. Intermittent behavior — Other users have reported this same error occurring intermittently (e.g., some transfers failing while others succeed within the same timeframe) . Recommended steps: - Check the SIP connection log in your call logs for the specific failure reason [(1) ]() - Note your call ID so support can investigate - If using a SIP trunk provider (Telnyx, Twilio, etc.) , the issue may be on their end — contact your provider with the error details [(2) ]() If you're using custom telephony, verify your trunking settings and termination URI are correctly configured. To get specific help, create a ticket in the bug-report channel with your call ID so the team can investigate. [(9) ]()

​ To learn more, see the sources I considered: [[1] Debug call transfer failure](https://docs.retellai.com/reliability/call-performance) [[2] 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 Since you're using a Retell-provided number, the transfer failure is still likely a telephony-layer issue. Even Retell numbers use underlying providers (Twilio or Telnyx) . [(1) ]() , [(2) ]() Things to check: 1. Caller ID configuration — If you have "User's Number" selected as the displayed phone number for transfers, caller ID override may not be supported depending on transfer type. Retell Twilio numbers support showing the user's number on both warm and cold transfers, but Retell Telnyx numbers only support this on cold transfers via SIP REFER. If caller ID override is not supported, the transfer would fail. [(3) ]() 2. Transfer destination format — Ensure your transfer number is in E.164 format (e.g.,

+14157774444

) or a valid SIP URI. 3. Human detection — If you have "Enable human detection" turned on for warm transfers, the call may fail if a human is not successfully detected (e.g., it was an IVR or voicemail, or the other party spoke after the detection timeout expired) . [(4) ]() Next steps: - Check the SIP connection log in your call logs for the specific failure reason - Note your call ID and create a ticket in the bug-report channel so the team can investigate the intermittent behavior Since this was working minutes ago and suddenly stopped, it may be a transient issue with the underlying telephony provider that the Retell team can investigate with your call ID.

​ To learn more, see the sources I considered: [[1] Create Phone Number](https://docs.retellai.com/api-references/create-phone-number) [[2] List Phone Numbers](https://docs.retellai.com/api-references/list-phone-numbers) [[3] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[4] Debug call transfer failure](https://docs.retellai.com/reliability/call-performance) ​

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

### Answer Glad you were able to fix it! That's correct — the caller ID override behavior differs between Retell Twilio and Retell Telnyx numbers. [(1) ]() For reference: - Retell Twilio numbers support showing the user's number on both warm and cold transfers - Retell Telnyx numbers only support this on cold transfers via SIP REFER - If caller ID override is not supported for your configuration, the transfer will fail Thanks for sharing the solution — this is a common gotcha that can help others troubleshooting similar transfer failures.

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

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