We’ve already knew that some sort of integration is in the pipeline, but now we have the clearest proof yet in the form of an unfinished feature that can be accessed via a debug menu in the Hangouts app.
Google Voice may soon be integrated with Google Hangouts
According to a report quoting multiple sources, Google's unified messaging service will be dubbed Babble. Combining the functionality of Google's many communication apps, Babble will likely be shown off at Google I/O.
Mystery app is not the new unified messenger; it’s an internal testing app