Gmail added a long-awaited feature: you can call any phone right from Gmail. When you install the voice and video chat plug-in, "you can call phones in the US and Canada for free. You can also call in other countries, but you'll have to pay. Fortunately, Google's rates are really low and the service is cheaper than Skype — (comparison table) with calls to U.K., France, Germany, China, Japan—and many more countries—for as little as $0.02 per minute," informs Google.
To get started, check the box next to Google Chat in your list of forwarding phones and the next time someone calls your GV number, Gmail will notify of the incoming call. You can take the call or listen in a message.
To make a call, just click Call phone link in Gmail and enter any number or name from your address book.
All calls made from Gmail will display your GV number as the outbound caller ID and all international calls will use your Google Voice calling credit. You can also receive calls made to this number right inside Gmail (instructions).
If you're using Google Apps for school or business, then you won't see it quite yet.
More Info: Gmail Call