Sadly, Google hasn’t made it easy to use their voice system. Up until recently we have only been able to make call from Google Plus but now you can do so from hangouts on Gmail right from your PC! You can also make calls straight from the Google voice website. We may say a complete combination of Hangouts and Voice but as of yet it is simply a rumour.
The rumor is that eventually we're going to see Google Voice merged completely into Hangouts, and that's getting even closer to becoming reality. You can now make phone calls via Hangouts directly from the Google Voice website, no Google+ necessary. As Mountain View's Alex Wiesen notes on G+, the new feature is listed as an option in the "phone to call with" drop-down box. It seems a bit minor, sure, especially when you consider that Hangouts is already how you make phone calls from within Gmail -- but any updates for the oft-neglected service are welcome, right?