For some maybe it is an attempt. I have worked with AdWords ads with phone numbers in them and they have a higher click through rate than other ads. Not by a lot, but it seems consistant.
As a seller you may think you might get something for free. As an AdSense advertiser, you may think you are giving something away for free.
But as a shopper looking on the internet - would you EVER pick up the phone without clicking an internet link first? I know I wouldn't.