I have a repeatable issue and can only say it appears to be a bug or glitch with the current version of 3CX. I don't know if it has been an issue for awhile but I only identified it today and it occurs every time. My best guess is it has something to do with how 3CX web interface saves sip/voip provider information when adding new providers. The issue is after adding a new SIP/VOIP provider, a completely separate and working sip trunk now reports proxy auth required and 404 user unknown. The only fix for after it breaks incoming calls for the unrelated sip trunk is to remove the just added trunk and reboot the server. Wire Shark reveals specially what happens, even though I have no idea why 3cx alters incoming call responses on one sip trunk when adding a new one. I can only assume it's unintentional or a bug. Here is how it occurs, maybe someone at 3cx can explain why this breaks and if a fix or workaround is available. I can make a call from the 3rd party PBX over a SIP trunk to 3CX. No issues occur and calls complete successfully. Immediately after adding another sip trunk as a new VOIP provider on 3CX, 3CX now reports to the 3rd party PBX for incoming calls a 407, Proxy Authentication Required on the unrelated existing sip trunk. It then goes on to report 404 User unknown for all incoming calls on the unrelated sip trunk from the 3rd party PBX. Rebooting the server does not resolve the issue. Only removing the just added sip trunk and rebooting fixes it. Outgoing calls have no issues from 3cx to the third party PBX throughout this issue. Below are the overview wiresharks, the only difference between them is the working one was before adding a new sip provider and the broken one is after. Taken within a minute of each other. I performed this 3 separate times, each time adding the new sip trunk, seeing the break, removing it, reboot, and getting it back to working again. I have thought to re-make the 3rd party PBX trunk on the 3cx side, the problem with that is I have about 100 inbound rules that I do not want to have to remake. I'd rather be able to just add an additional sip trunk without it breaking incoming calls on another one. ***Update Edit*** Well, after doing some more research and testing some things out, while this remains a constant issue, a fix/workaround is pretty simple. After adding the new trunk, if I select the broken trunk that 3CX is responding of proxy auth and unknown user, edit it, then back out again re-saving it, it fixes the issue and 3cx responds properly to the 3rd part PBX. So bug/glitch, yes, but fix/workaround is pretty painless.