What It Is:
AdWords call metrics is a new feature that lets you measure the number of phone calls generated by your AdWords campaigns. Using the technology behind Google Voice, call metrics creates a unique phone number for each of your campaigns which is automatically inserted into your ad when it appears on Google.com, on both desktop computers & high-end mobile devices.
When a potential customer calls the number in your ad, AdWords notes that the call took place. Then, when you view your AdWords reports, you'll see the number of calls you received and other metrics, including call duration.
Why You'd Use It:
With insight into how many calls your ads are generating, you’ll get a better understanding of the value that AdWords is bringing to your business. At the same time, featuring a phone number in your ad will give your potential customers another way to reach you.
Availability
Call metrics is currently available to a limited number of advertisers. If call metrics is available in your account, you'll find this option in the 'Phone extensions' section of your 'Ad extensions' tab.
If call metrics is not available in your account and you'd like to use this feature, please sign up here. We plan to make call metrics available to more advertisers in the coming months and will let you know when it's available in your account.
Source: http://www.google.com/ads/innovations/callmetrics.html