That’s frustrating, especially with a production go-live blocked. From your trace, this looks less like a 3CX issue and more like missing inbound Digest auth on the Vapi SIP side (realm is there, but no credentials mapped, so 3CX sends an empty username and gets a 500). Usually this needs either proper SIP auth credentials or an IP allowlist tied to that Vapi SIP URI, which isn’t clearly documented yet. I’ve helped fix similar 3CX SIP auth loops and can help you get this working fast. Quick question, are you aiming to use Digest auth or IP whitelisting from 3CX? Send me a DM, easier to go deeper on this privately with your SIP traces. @Marc Bergmann