Bug report attached — the inline text kept getting auto-flagged, so I'm posting it as a file. Warm transferCall to PSTN fails on a BYO Elastic SIP Trunk in the IE1 region. https://cdn.discordapp.com/attachments/1507006404907499720/1507006405272535151/vapi-ie1-transfer-bug.txt?ex=6a105495&is=6a0f0315&hm=da88a12c755af49199e1d9261da4715cf5df1638760527055e56fd3872d2f942&

Thanks for the detailed report and the reproducer, this made debugging a lot faster. I've gone through the logs for call 019e4a89 and confirmed the root cause: your Twilio account is on the IE1 region, but Vapi's transfer routing currently only works with US1. When the PSTN transfer is attempted, Vapi calls the US1 Twilio API endpoint, which can't find your IE1 call, so it 404s and the call drops with [call.in](http://call.in)-progress.error-transfer-failed. The fix: switch your Twilio Elastic SIP Trunk to US1 routing. Once your trunk is on US1, warm PSTN transferCall should work as expected and you can drop the cold-callback workaround. Let me know if you run into anything while making the switch and I'm happy to help. Regards, Chiranjeet Vapi Support

Thank you for the answer! Quick follow-up — checked our setup, and our Trunk is already on US1 (Active Configuration: "United States (US1)", visible under the US1 region tab in the Twilio Console; the IE1 tab shows zero trunks for our account). Other details: Account API base is api.ie1.twilio.com (account region is IE1) Phone number is +319… (Dutch mobile) Trunk: US1 Given that the trunk is already where you suggested but warm PSTN transferCall still fails, could the actual lock-in be elsewhere? Two options we considered: a) The mismatch is on the account region (IE1) vs. Vapi's transfer routing (US1), not on the trunk. b) Should we instead create an IE1 trunk to match the account region + NL number? Or is there a third path we're missing? Happy to make whatever change you'd recommend.

Hey, apologies for the delay on this! Good news, we have a definitive answer. It's not your account region or your trunk that needs to change, it's the routing setting on the phone number itself. Here's the fix: 1. Go to Twilio Console > Phone Numbers > Manage > Active Numbers 2. Click your +319... number 3. Go to the Configure tab 4. Under Voice Configuration, find the Routing section 5. Switch it to United States (US1) Once that's set to US1, warm PSTN transferCall should work and you can drop the cold-callback workaround. Let us know how it goes! Regards, Chiranjeet Vapi Support

Hi Chiranjeet, it worked, thank you very much! You are the best!