why am I getting this error Retell AI #❓・ai-evy-support

why am I getting this error

01/27/2026, 2:36 PM

@AI Evy why am I getting this error? never had it before when transfer node was used. error: Error transferring call via SIP REFER, SIP status code: undefined SIP error category: unknown Error: twirp error unknown: server not found

AI Evy

01/27/2026, 2:36 PM

Thanks for tagging me @Q4. I'll look through relevant sources and get back to you shortly.

AI Evy

01/27/2026, 2:37 PM

### Answer The "server not found" error with SIP REFER typically indicates an issue with the transfer destination or telephony configuration. Here are the most likely causes and things to check: Common causes: 1. Invalid transfer number format – Ensure the transfer number is in correct E.164 format (e.g.,

+14157774444

) or a valid SIP URI in the format

sip:username@domain

. [(1) ]() 2. Telephony provider issues – The error "server not found" suggests the SIP endpoint for the transfer destination couldn't be resolved. This could be a temporary issue with your telephony provider or the destination's SIP infrastructure. [(2) ]() 3. Caller ID configuration with Telnyx – If you're using Retell Telnyx numbers with warm transfer and "User's Number" as the displayed caller ID, the transfer will fail. Retell Telnyx numbers only support User's Number caller ID with cold transfer using SIP REFER. Recommended steps: - Verify the transfer destination number/SIP URI is correct and reachable [(1) ]() - Check if you're using warm or cold transfer, and ensure your caller ID settings are compatible with your number type [(2) ]() - If the issue persists, create a ticket in #❓・bug-report with your call ID so the team can investigate the SIP connection logs [(3) ]() This error has been reported by other users recently with similar undefined SIP status codes, and the Retell team has been investigating these cases. [(4) ]() , [(2) ](ht

AI Evy

01/27/2026, 2:37 PM

tps://discord.com/channels/1186424376300679209/1463968434646679604)

AI Evy

01/27/2026, 2:37 PM

To learn more, see the sources I considered: [[1] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[2] Transfer Node Error](https://discord.com/channels/1186424376300679209/1463968434646679604) [[3] Discord message from @mauvatar on 2026-01-19](https://discord.com/channels/1186424376300679209/1294891368505937952/1462849273413632221) [[4] Transfer Node - Not Transferring](https://discord.com/channels/1186424376300679209/1404477786986512495)

AI Evy

01/27/2026, 2:37 PM

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

AI Evy

01/27/2026, 2:45 PM

Thanks for the follow-up, @Q4. I'll get back to you soon.

01/27/2026, 2:45 PM

@AI Evy transfer is successful and it hits the endpoint - phone rings 🙂 so where is the problem?

AI Evy

01/27/2026, 2:46 PM

### Answer If the transfer is successful and the phone rings at the destination, the error message you're seeing is likely a logging/reporting issue rather than an actual functional problem. The "SIP status code: undefined" and "server not found" error may be appearing in your logs even though the transfer completed successfully. This type of discrepancy has been reported by other users. In one case, a user experienced SIP REFER behaving differently and simply republishing the same agent version resolved the inconsistency. [(1) ]() A few things to consider: - The error may be a false positive – If the call is actually transferring and the destination phone rings, the core functionality is working. The error logging may not accurately reflect the outcome. - Check the
disconnection_reason
field – After the call ends, you can check if

disconnection_reason

call_transfer

and whether

transfer_destination

is populated correctly in the call data. [(2) ]() - Republish your agent – One user with a similar SIP REFER issue found that republishing the same agent version fixed unexpected behavior. [(1) ]() If the transfers are consistently working despite the error message, you may be able to treat this as a non-blocking logging issue. However, if you want the Retell team to investigate the error message itself, providing a call ID in a bug report would help them look at the SIP logs.

AI Evy

01/27/2026, 2:46 PM

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

AI Evy

01/27/2026, 2:46 PM

To learn more, see the sources I considered: [[1] SIP REFER - To Telnyx](https://discord.com/channels/1186424376300679209/1464386925665194047) [[2] Update Call](https://docs.retellai.com/api-references/update-call)

Previous Next