I am having problems with a spa 400 gateway connected to our 3cx server on windows.
I can make outgoing calls from our phones but i can't get incoming calls.
While no longer "officially supported" here is the link to getting the 3102 to work.
http:/ /www.3cx.com/voip-gateways/linksys-spa3102/ - NO LONGER AVAILABLE
Probably some tips you can use in there.
And here is a link to a site where they set up a SPA400 for use with 3CX.