As of today, you can use Gmail to make phone calls. According the Official Google Blog, using your computer’s microphone and speakers, you can employ Gmail voice and video chat to make calls to any phone.
"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," the blog states.
Dialing a phone number works just like a normal phone. Just click "Call phone" at the top of your Gmail chat list and dial a number or enter a contact’s name. If you have a Google Voice phone number, calls made from Gmail will display this number as the outbound caller ID. You can also receive calls made to this number inside Gmail.
U.S. Gmail users should see the "Call Phones" item in their chat lists within the next few days, and must install the voice and video plug-in to make calls. Non-U.S.-based users and Google Apps school or business users will not have the calling capability yet, but the Google blog states, "We’re working on making this available more broadly — so stay tuned."