Click-to-Call For Smartphones Are Calling Advertisers,

Google Click to Call for smartphones are in a swing and swinging advertisers their way. So are the users, who realize the value of having a click2call number included in an advertising campaign. According to the two posts below, Click-to-Call for smartphones has come a long way, with a little push and support from Google. But the Click-to-Call ads on both search and the Google Display Network are generating millions of calls every month on mobile phones and driving strong performance for advertisers.
If you are an advertiser, it is pretty easy to add Click-to-Call for smartphones to your campaigns;

• Location extensions -- display your local business phone number to nearby users on mobile devices with full browsers. View instructions for enabling location extensions.
• Phone extensions -- display your national or vanity phone number to all users on mobile devices with full browsers. View instructions for enabling phone extensions. 

Official Google Blog Via Click-To-Call For Smartphones, How Googlers Do It