I am curious to know how you were entering numbers when placing phone calls while using GV. My understanding is that in order for the GV service to be used you must enter a 10 digit phone number i.e the area code must be included when dialing. I did some testing and found that when I dialed using only 7 digits my Sprint number was used but when I used the 10 digits my GV number was used.
When I first got my GV number and tested it, I called my mom a few times. She is in another state, so I know I was using the 10 digit phone number. What I would do was check my minute usage with Sprint, then call her via GV and talk for about 10 minutes. After hanging up, I would then go check my minute usage again and it would show I'd used 10 minutes of call time. Also, when doing international calling it counts against my 450 minutes as well.
By the way, how were you verifying whether you were using your cell or GV number?