You're both talking about two different things which is one of the issues with G-Voice. It's kinda all over the place....
The G-Voice App will let you mask your G-Voice number when dialing out on your Cell line.
The Hangouts Dialer will let you make VOIP calls. Receive if you turn it on in the Settings.

It would also be nice if the Hangouts Dialer could integrate into the Android Dialer like GrooveIP could.
Heck you can make 3 way calls in the Desktop version of Hangouts. Guess how you do this? You think it's in the Desktop Hangouts app? Nope....
You have to goto Google Plus. Click on the Hangouts Icon then click start video Hangout! Then you can select add Telephone.... Hopefully this gets added to the Desktop and Mobile app....
Google Voice and Hangouts has a lot of cool features and ideas.... It's just kind of a mess....
Depending on what I need to do with Google Voice on my phone I might need to switch between Hangouts, Google Voice App or the Desktop version of the Google Voice site....
The Google Voice site should be retooled to work hand in hand with the Hangouts chat app. It's easier to find messages and voicemails on the G-Voice site and access settings but it needs a UI overhaul too.
Hopefully this survey helps....