Use GV as primary number

[teduzzo5]

Hi there! I'm going to move to the U.S. and since mobile phone is really expensive I made a "system" to keep mobile phone cheap.
Could you please tell me what you think and if you think it's going to work?

First of all I will buy a paygo sim (perhaps an ATeT MNVOs paygo), and a google voice number (which should be free).
I will give everybody my google voice number and not my ATeT number.

When I call someone:
If I'm at home (or in a place with free wifi) --> I'll use GV from my computer/phone (free)
If I'm without wifi--> I'll use my ATeT phone (paygo rates will apply)

If someone calls me:
If I'm at home (or in a place with free wifi)--> I'll receive the call on my computer/phone (free)
If I'm without wifi--> I will active "calls forwaring when I'm offline" so the call will be diverted to my ATeT number and I'll get (pago rates will apply)

Furthermore when I'll be outside the U.S. I will forward the calls to my European phone number (paying just a little because GV rates are about 2-3cents per minute and incomping call are free in EU)

What do you think? Is it going to work properly?
I have a Samsung Galaxy S1 and a Nokia Lumia 920...what apps could I use to use GV on my phones?

Thanks in advance