Whether you were enjoying free domestic calls via Gmail’s built-in phone function throughout 2012 or this is the first you’ve heard of the feature, you can take advantage of free calls throughout 2013.
From the official Gmail blog:
Many of you call phones from Gmail to easily connect with friends and family. If you’re in the US and Canada, you’ll continue to be able to make free domestic calls through 2013. Plus, in most countries, you can still call the rest of the world from Gmail at insanely low rates.
Free calling within the US and Canada through 2013 [Official Gmail Blog]