I've been using GV+SIP on my N900 for awhile now and it seems to work ok. I thought I would try ditching the whole "minutes" plan altogether and I changed to a data-only plan.

Now when my phone rings, I have to remember to check if its a regular phone call or a SIP call. If it is SIP, I can answer. If it is regular phone call, I have to call them back over SIP.

I was able to block SMS and MMS messages through the carrier. Now the question is, how to I suppress ringing on essentially all calls that are not SIP calls?

EDIT: Nevermind, T-Mobile has changed the way data plans work. Now data-only plans don't deliver phone calls at all. They used to go through at something like $0.25 per min.