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I created the app HangoutNow to do this. Due to the way Hangouts is designed, you still have to press the Call button in Hangouts Dialer to start the call. I am working on the next version of the app which will most likely be able to press the button automatically.
Hangouts Dialer can place Google Voice Calls and I created this app that integrates Hangouts Dialer into Google voice search and some third-party apps.
HangoutNow (I made it) lets you call with Hangouts from Google Now
HangoutNow (I made it) lets you call with Hangouts from Google Now