My sip trunk provider (Telnet Worldwide in Michigan, telnetww.com) is complaining because my 3CX systems responds with a "407 proxy authentication required" every time they send an INVITE for an inbound call. It doesn't happen for the main trunk line but for the extra DIDs. My guess is 3CX (version 12 sp6) sees that the main number is authenticated and lets that through but challenges the sub-lines (various DIDs) are they are not authenticated. TelnetWW is saying their "new server" can't handle it and I will have to stop. I have played with all settings of "Require Registration" in the voip provider setup to no avail. I can see the traffic with wireshark so I know 3cx is sending it. I really am fine with trusting anything from my providers ip address but can't figure out how to turn off the challenge. Any ideas?