If you have upgraded from Talk to Hangouts, outbound calling via Gmail no longer works (there is no longer even an icon for it) since the introduction of Hangouts. So that's a change (though, a reversible one, since you can switch back to the old GTalk Chat interface in Gmail.)
Google's stated that they intend to fully integrate outbound calling via Hangouts (in fact, it sounds like their intent is to replace the entire distinct Voice functionality, including the UI, by integrating Voice fully as a subfeature of Hangouts), but they haven't done it yet.