Google Voice uses local access numbers to connect you, so your minutes are used for both incoming and outgoing calls. That said, you can usually add your Google Voice number and the local access number to your carriers "circle" (or whatever they call it) and the calls won't count against your minutes. Doing this for incoming calls requires additional configuration in Google Voice - presenting your GV number as the Caller ID for incoming calls. The down side to this is that you lose your ability to see who's calling when they call via your GV number.
