You can set up a Google Voice number to work with your device's phone app and line, but you still need the Voice app to set that up. For message/voicemail access, the only place to do that is the Voice app (or calling yourself from your number to access voicemail). There are messaging apps that support adding a Google Voice account (like Rambox), but they're basically just pulling the web version of the service and displaying it into a single app with your other web services).