MCD version wont make a difference here, as the 100 trying, 180 ringing, etc is part of the SIP RFC standards, so versions of software wont be the cause.
You mention that the MBG is returning a 100 trying. Is this what the provider is seeing, or are you seeing it on the MBG?
If you run a tcpdump on the MBG and the 3300 what is seen on each? Also assuming that the voicemail pilot answers when it is called internally, and that the same happens with DID calls in to normal handsets as well?