Now they extended the free calls offer till the end of this here so, now users can enjoy the free calling till the end of year 2011,
Here the official announcement from the Gmail blog.
When we launched calling in Gmail back in August, we wanted it to be easy and affordable, so we made calls to the U.S. and Canada free for the rest of 2010. In the spirit of holiday giving and to help people keep in touch in the new year, we’re extending free calling for all of 2011.
In case you haven’t tried it yet, dialing a phone number works just like a regular phone. Look for “Call phone” at the top of your Gmail chat list and dial a number or enter a contact’s name.
To learn more, visit gmail.com/call. Calling in Gmail is currently only available to U.S. based Gmail users.
Happy New Year and happy calling!