Source: http://www.engadget.com/2013/05/20/hangouts-upgrade-disables-outbound-google-voice-calls-in-desktop/
Anyone who's tried out the new Hangouts feature in Gmail in place of the traditional Google Talk option got a bit of surprise when they went to make an outbound call from their computer. That feature gets disabled when the new Hangouts replacement is switched on, leaving folks looking to make calls with no other option than to revert to the old Google Talk in Gmail or seek out another alternative. That's apparently just a temporary change, though, as Google's Nikhyl Singhal has now confirmed that both outbound and inbound calls will "soon be available." Unfortunately, he's not getting any more specific than that for the time being. He did, however, reiterate that "Hangouts is designed to be the future of Google Voice," and that "making/receiving phone calls is just the beginning," further adding that "future versions of Hangouts will integrate Google Voice more seamlessly."
Source: Nikhyl Singhai (Google+)