Google has just announced today the Google Voice facility straight from your Gmail which would be rolled in upcoming days. This service will let you call on phones of your friends. All you would require is this plugin
Watch the video below straight from Google to understand the service
From Google’s Official blog
Calls to the U.S. and Canada will be free for at least the rest of the year and calls to other countries will be billed at our very low rates. We worked hard to make these rates really cheap (see comparison table) with calls to the U.K., France, Germany, China, Japan—and many more countries—for as little as $0.02 per minute.
Calling a number would be no biggie. Just like your usual phone works. Just click on Call phone and dial a number
If you have Google voice phone number, that will appear as your ID on outbound calls while you can also recieve calls on the same number straight from your Gmail.
Only launching for US users for now and will be rolled later for rest of the world.