I hate to open a "closed" case but I think this could use some more clarification. 

Google voice is a great product that might give the OP the features desired even with a new number. Because it is free to start a new number with GV, I would recommend doing that first and setting it up to run on your Android (or other supported) devices as desired. I've had GV running on 3 different devices, even 2 at a time, without changing my current mobile numbers with VZW. Doing so still allowed multiple phones to ring at the same time and allowed for a universal catch-all for voicemails (IE, they could call any of my numbers and I would be notified through the app, with VM transcripts, on whatever handset I had with me). I can understand the advantage of porting YOUR cell phone number to the service but considering the pending ETF I don't think it would be worth it at this time. Once it's out of contract it might make more sense, but I would put some time in with a free GV number first to make sure the service does what you expect. 

adambomb might be on to something in his post, but it's too risky for me to recommend...

The contract is on your mobile number. You can change your plan any time, including your data plan by activating a "dumb phone". You can change your number as suggested, but then you run the risk of it being lost in a holding pattern. And it still won't remove the contract on that number. adambomb's solution can work in theory, but I would have to say you would need extreme luck for everything to go your way. If your number is important enough to you to want to port it to GV, then I wouldn't gamble with it. 

Most importantly, you still need a cell number in addition to a GV number. Google is not a phone provider. Yet. 

My 2¢