Google has been testing click to call features for a while, but the true value to having Google Audio Ads, which was discontinued recently, was to get the toll free and trackable free phone feature. In fact, many AdWords customers used it to track their campaigns, even without using audio ads.
Since Audio Ads was discontinued, the phone tracking was discontinued as well. As of May 5th, Google has discontinued the service. If you read the Google help page, it reads:
Important news about Google Audio Ads: On February 12, 2009, we announced that Google will discontinue the Audio Ads platform. All audio ads will stop running on May 31, 2009.
Also on this page, it says:
Call reporting numbers are being discontinued in AdWords Business Pages on or shortly after May 5, 2009.
So if you have been using these toll-free numbers, make sure to swap them out to new numbers.