Today's launch also improves the desktop calling experience in a number of ways. For example: you can add multiple phone numbers and video participants to the same call; and you can play sound effects (like applause or laughter) with the Google Effects app.
To make a call from Hangouts, just look for the new phone icon in Gmail, or for the new "Call a phone" menu item in Google+ and the Chrome extension. And of course: if you haven't yet tried Hangouts in Gmail, you can always click your profile photo in the chat list and select "Try the new Hangouts."
Making calls from Hangouts will rollout over the next couple of days.