Source: http://lifehacker.com/google-voice-users-can-now-answer-phone-calls-in-hangou-509395930
Gmail users got a taste of Google's new Hangouts feature
If someone calls your Google Voice number and you're signed into Hangouts, you'll be able to answer the call right then and there. It'll open up in its own window, just like a video hangout, and you'll be able to talk to them as if you were talking to them on the phone. It doesn't look like you can make outbound calls yet (typing in someone's phone number just opens up a text chat with them), but it seems like Google's trying to integrate the Voice features we had in the old chat. Head on over to Gmail to give it a try!
You Can Now Call Into Google+ From Regular Phones | Disruptive Technology