About 6 months later than rumored, but it's finally here! It looks like there's still some work to be done, though. I never got prompted about SMS like in a screenshot I've seen, but it seems the functionality will roll out in a few more days? Here are a couple articles about it
Hangouts 2.3 Update Brings Remaining Google Voice Integration And A Big Visual Overhaul [APK Download]
Google Hangouts Gets Google Voice Integration And Free VoIP Calls | TechCrunch
Hangouts 2.3 Update Brings Remaining Google Voice Integration And A Big Visual Overhaul [APK Download]
Google Hangouts Gets Google Voice Integration And Free VoIP Calls | TechCrunch