Users can now call friends on their phones from inside the hangout and have them join the conversation. It already added to gmail a while ago and the same feature is now avaliable inside Hangouts. It only works for calls tothe US and Canada. Maybe more countries added later.
Google Jarkko Oikarinen wrote on Google+”We are constantly listening to feedback so that we can make Hangouts even better for Google+ users, and we’re excited by the really cool ways people are using the product. Today we’re making it possible to make phone calls from inside a hangout, so you can dial people into the room from wherever they’re at.”
This new feature is initially available inside Hangouts with extras (plus.google.com/hangouts/extras), and we currently support calls to the US and Canada (for free, in fact).