Google has extended Gmail free calling to US and Canada for all 2011. Calling a phone number via Gmail is quiet easy; you can call the phone number in the same way as you call your friends in Gmail voice and video chat. Look for “Call phone” at the top of your Gmail chat list and dial a number. And when it comes to free phone calls, then why not to call the phone instead of waiting for the friends come online.
In case you haven’t tried it yet, then visit this page http://www.google.com/chat/voice/
For more information read official Gmail blog