When a phone is configured to use Google Voice for voicemail, his does that work? is the call routed to VM by the phone, or is the setting changed at the carrier too?
It's routed by the carrier so even if your phone is off it will get appropriately routed to Google Voice. With non-android phones you have to type the carrier specific activation code into the keypad and hit send to make the change at the carrier level. The Google Voice Android App takes care of this for you with a simple toggle in the settings menu.