tuxychandru has it right.

From the article: "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."

And: "If you’re not a U.S. based user—or if you’re using Google Apps for your school or business—then you won’t see it quite yet."

Also, you might want to check out the rates (they do seem pretty cheap for international calls, but I'm not an American so I don't know the standard): https://www.google.com/voice/rates,