that makes sense. I haven’t tested it from a separate SIP client yet, but I did find one useful comparison point: another user here reported successful transfers after switching from “Dial to SIP URI” to Elastic SIP Trunking using Asterisk, including transfers back to internal PBX extensions. That seems to suggest at least some SIP stacks can handle the 407 challenge/auth flow correctly in a similar trunk-style setup, though it wasn’t Zadarma specifically. I’m happy to test directly against Zadarma with another client to isolate this properly. What would be the simplest way to reproduce Retell’s behavior: an unregistered UAC sending an outbound INVITE over the trunk, receiving a 407, then retrying with trunk credentials? Would a quick pjsua / sipsak command-line test be enough, or is there a simpler tool you’d recommend for someone without a deep SIP/dev background?