Google Voice doesn't provide any sort of Voice service on it's own. It uses existing voice services (landline and cell phone) to function. To place a Google Voice call, you log into Google Voice and specify which of your existing voice services to use to place the call (i.e. landline, cell phone, VoIP Account (only Gizmo5 at the moment). You tell Google Voice (GV) which telephone number your wish to call. GV will initiate a call to the number your specified, and GV will initiate a call to your designated voice service. Once your and the party you called pick the call is routed though Goggle's extensive fiber network. GV acts as the middle man for the call.

So for example, I could use my N900 Cell Number as the endpoint of a GV call, or I could use my Gizmo5 account (SIP VoIP) as the end point. And I could be logged into Gizmo from my desktop or I could be logged into Gizmo from my N900 over a WiFi or cellular data connection. The later is what scare Cellular service providers are your don't burn any cellular minutes, it's strictly a data connection.