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 very low rates. Google worked hard to make these rates really cheap with calls to the U.K., France, Germany, China, Japan—and many more countries—for as little as $0.02 per minute.
Dialing a phone number works just like a normal phone. Just click “Call phone” at the top of your chat list and dial a number or enter a contact’s name.
Original post: 
CNET has learned that Google is testing a Web-based service within Gmail that will allow users to place phone calls from their in-boxes. It’s launched from the Google Chat window on the lower left-hand side of a Gmail page and allows users to place and receive calls from within their contacts through a user interface that strongly resembles the one used in Google Voice.
