Source: http://www.engadget.com/2013/07/09/outbound-calls-from-desktop-gmail-are-back-with-free-calling-to/
Today Google has brought back the ability to make phone calls from desktop Gmail, Google+ and via its Chrome extension, and also announced that calls to the US and Canada are free (from countries where Hangouts calling is available. The feature disappeared when Google introduced its new Hangouts setup, but a post on the Gmail blog claims that now it's better than ever. According to the company, "Hangouts is designed to be the future of Google Voice," with making and receiving calls just the beginning. For now, users can enjoy new features like adding multiple phone numbers and video participants to the same call, and using sound effects from the Google Effects app. Current Hangouts users should see a new phone icon in Gmail, or a "call a phone" menu item in Google+ and the Chrome extension.
Source: Gmail Blog