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The setup is quite simple and means that all advertisers running mobile ads can add a phone number via the adwords phone extensions menu (make sure to select the "iPhones and other mobile devices with full Internet browsers" campaign option as well).
Following setup, advertisers with phone numbers associated with their ads will gain access to data on how many calls they receive per keyword, ad group or campaign.
On the user side of things, iPhone, Android and Palm Pre device owners should start noticing the national and local Google ads with click-to-call phone numbers at the bottom.