According to gmailblog Free calling within the US and Canada through 2012.
As the holiday season approaches, we’re happy to announce that we’ve extended free domestic calls within the US and Canada for 2012. This is our way of helping you connect with friends and family across the country. And you can still call the rest of the world from Gmail at our insanely low rates.
Once again, Google will extend free Gmail calls for another year, which will allow desktop users to call any domestic number within the United States or Canada through the end of 2012.
To initiate a call, Gmail users can click on the “Call Phone” option from the chat menu on the left side of the screen, and then dial any domestic number. Google Voice users can also accept incoming calls through desktop Gmail at no charge.
The main difference between Gmail and Skype is that Google allows users to call mobile phones and landlines for free, whereas Skype only allows free calls to other Skype accounts. Skype, however, offers mobile apps for calling other users.
Google doesn’t offer any free voice calling on mobile phones. Both services charge for international calls.