Sorry, I probably mislead you!
Mine was a brand new install of 3.5 where I restored a backup taken under 3.0. It was an “upgrade” in terms of going up a version only, not that I installed 3.5 over 3.0.
In your brand new install do you have any forwarding under virtual server at all? Any chance 5060 or the RTP ports are forwarded to a wrong host?