A little late to the party, but here are the answers...
Google Voice IS NOT VOIP. Google voice is simply a "cloud" number that supports call forwarding. When you make a call using the Google Voice app, the app calls YOUR Google Voice number, which then forwards the call to your intended destination. It works the same for receiving calls. When a person calls you, the call is then forwarded to your actual phone number.
Therefore, as long as you get free calls to/from your Google Voice number, all calls (to any number) are free. Assuming you use Google Voice exclusively for incoming and outgoing calls, that is.
@Dun - Google Voice can not and will not use only a data connection to make outbound calls. Try removing all available networks (cell networks, not wifi) in your settings. This will disable the reception (your T-Mobile service) for your phone while leaving the rest of the phone operable. Then try to use Google Voice (make sure that you are connected to another data source, such as wifi). Google Voice will not work. If it wasn't dependent upon "plan minutes" it would work under this scenario.