It's finally here, Google's Hangouts App is now being updated with Voice Integration. Today, Google Voice is being rolled into Hangouts which will enable Voice SMS and voicemail via a popup in the conversation list. Go check your Hang-outs App Now.
Parts of this seems to be working fine, but others not so much. For example, messages sent to your Google Voice number will appear in Hangouts (mobile and desktop) just fine. Replies, however, appear to be busted in both locations right now. Presumably this is all just a bump in the road early in deployment. Of course, someone might have simply thrown the switch too soon.
Also note that if you turn on Voice integration, your Voice app will stop receiving messages-everything will go straight to Hangouts. The Voice website will also be updated with a notice that your texts and voicemail are being routed to Hangouts. The Voice Chrome extension will be able to send SMS, but it won't receive them anymore. Hangouts App should get another update during this week for full Google voice integration.