They most likely access whatever app you have set as default for those actions (Dialer & Messaging). This however, will never work with Google Voice, as it is not a Messaging application in the "normal" sense. Google Voice has a messaging feature, but that's all it is, a feature. Plus isn't not SMS in the traditional sense either; it just sends messages over the internet. In any case it's more of an IM feature than it is a SMS feature.