Ive been reading that there have been questions with Google Voice integration, using your sprint number, and mobile to mobile minutes. I was quite concerned with it myself, so I called yesterday and found out that if you use your sprint number to integrate with Google Voice, our mobile to mobile minutes are still free. Yes, I know that sometimes the customer service reps are...well...unreliable at times, so I took it upon myself to keep a transcript of the conversation. Here is the excerpt in case you are interested. Here is the quote:
Mobile to Mobile calling is being associated with your sprint number If you
are calling from your sprint number it will free for sure goggle voice service does not make anyeffect on this, Be assured.
If this turns out to be not the case, I at least have some evidence. Just thought I would post this in case anyone else had the same question.