Hi Retell team, I'm experiencing a persistent SIP 486 (user_busy / Rejected) error on ALL call transfers. The flow works perfectly — the issue is only at the SIP level. Setup: - Swiss numbers imported via Twilio Elastic SIP Trunk - Trunk: callora-retell.pstn.twilio.com - Agent ID: agent_6c8ed75ed78f82fd461c4b8a65 - Transfer destination: Swiss mobile numbers (+41xxx) What I've tested (all fail with SIP 486): - Warm transfer and cold transfer - Caller ID: both Retell Agent's Number and User's Number - Multiple destination numbers (tested 2 different Swiss mobiles) - Destination phones confirmed reachable (direct calls work fine) Twilio trunk settings (all confirmed): - Call Transfer (SIP REFER): Enabled - Enable PSTN Transfer: Checked - Geo Permissions: Switzerland enabled - Termination: Active with credentials Recent call IDs for debugging: - call_b30d90ab17eec447e646cb264f5 - call_f5760d5ed092f79e86f6efe5593 - call_ff9efaad30b7e385fdebbccdd58 - call_93e41f59a9ff253f335097ceb76 All return: error: Error transferring call, SIP status code: 486 SIP error category: user_busy Error: twirp error unknown: INVITE failed: sip status: 486: Rejected The transfer_call tool fires correctly with the right number, but the SIP INVITE is rejected immediately (within 2 seconds). The destination phones never ring. Is there a known issue with call transfers on numbers imported via SIP trunk? Or is there an additional trunk/Retell configuration needed for outbound transfer calls? Thanks!

Hi, I’ve seen this happen when the transfer INVITE gets rejected at the trunk level before it ever reaches the destination. Since the phones never ring, it’s likely a routing or outbound policy issue rather than the number itself. I can help dig into the SIP flow and Twilio trunk behavior for transfers. Quick question: is the transfer being sent as a SIP REFER or a new outbound INVITE from Retell? Also, is outbound dialing to +41 fully allowed on the trunk? @Francesco