Unlike jasonacg, I have had the opposite experience. And, unlike him, my Sprint phone number / GV phone number are not the same. I choose to keep them separate. I have my phone set to ask me each time I make a call if I want to use GV to make the call.
Each time I call through GV (even if I am calling someone's cell phone) my phone will dial GV's access number. Based on my Sprint bill, this access number is a land line, thus I don't get the benefit of free mobile to mobile calling.
Not sure if jasonacg is working different because he integrated the phone numbers together? I also agree with him about it being hard to find someone at Sprint to give you a straight answer. His advice to try and see is probably the best way to go...