If you go into Google Voice's options, under "Making Calls", you can select "Ask every time I make a call."
Google Voice calls operate on the phone network, not data. Additionally, it uses a different access number every time, to make sure you can't put it on any faves/friends&family/etc lists. One way to get around this is to use an app called Google Voice Callback. However, Google is quick to remove any app from the market that gets around Voice's minute usage. You can find it on the Amazon Appstore.