Yes this is doable via SIP REFER on Twilio Elastic SIP Trunking. Retell would need to trigger a SIP transfer and pass a valid sip: or tel: URI in the Refer-To header, and your Twilio trunk must have Call Transfers enabled. I’ve implemented similar Retell → Twilio PBX handoffs before—happy to help if you’re stuck on the REFER setup or TwiML side. @gphan.