I just stumbled upon testing of Click-To-Call icon on one of the Ads displayed in PPC
Just to test the service I feeded my nember and "bang" I was connected to Vendor over VOIP
I thought of playing a prank with my friend and entered his number... guess what? He got an unsolicited call from Vendor!
Moral of the story - There has to be some sort of verification before Vendor Calls the number feeded on Google SERP
May be a text message sent asking for verification (though these text message can be fun too) or may be some sort of permanent registration.
So you have never done market research before?
And you have never clicked a paid link with the intention of looking at that competitor's web site? I don't believe you for one second!
Also, the price for clicking on someone's ad is usually far below the price for a pay-per-call ad.