@Dolce, this is a late reply but you might want to check out this youtube video:
How to Keep your US phone number when you move to another country.
He explains how to port your number to Google Voice, emphasizing to do the actual porting at the last few hours before you leave the US. Otherwise your carrier will end your plan when you port your US number away from them. Lots of other good (and sometimes confusing information). And be aware that the porting must be initiated from the US. I started the porting process in the US but it wasn't working. Finally got it straightened out here in Portugal. The porting successfully completed here, but only because I had initiated it in the US.
I followed his plan (from the video) and now am able to make calls and texts to the US through Google Voice and my number is preserved through Google Voice. I'm not enough of a techie to understand it all, but I managed to get it to work.
Another youtube video that was helpful to me was How to port your number to Google Voice.