Google announced a new ad tracking metric for tracking calls or click to calls triggered from mobile landing pages. The new metric is fully integrated into your AdWords campaign and gives you just a bit more useful information for your advertiser efforts.
You'll now be able to attribute clicks on your phone number or 'call' button back to the AdWords campaign, ad group, ad or keyword that brought a customer in. As this is a new tracking metric, there won't be new charges or changes to CPCs. We hope that this new metric will give advertisers and agencies new, richer information on the value and returns from their mobile advertising.
For technical details on how to set up call tracking, see this help document.
Greg Sterling has his analysis of this at Search Engine Land.
Forum discussion at Search Engine Watch Forums.