I am currently trying to get some NEC SIP phones to register with 3CX. The NEC handsets are DT700's. Very little info out there on them. When I watch a registration take place, the phone sends a SIP registration request to 3CX, and 3CX responds with a 407 Proxy Authentication Required. Then that's it. The phone never responds back. I'm guessing that the phone is looking for 401 response from 3CX so it is just completely ignoring the the 407 response. After googling a bit, I came across this: "Note: The IPKII/SV8100 will not recognize a 407 Unauthorized message. Must be a 401." So it looks like that is probably the case. If that's so, is there anything at all I can do on the 3CX side to get these things to work? Thanks.