http://www.google.com/help/faq_clicktocall.html
I'm not so sure this is a good idea...
Awesome! :D
I wonder how that's going to turn out...
Not sure if they just added this, but:
Quote#5 What if someone enters my phone number instead of theirs as a prank call?
Google takes fraud and spamming very seriously. We use technical methods to prevent future prank calls from the same user within a reasonable period of time. You won't be charged for any such calls. Please contact adwords-support@google.com if you believe someone is entering your phone number without your permission or knowledge.
They take both parties' privacy very seriously. They may do things to prevent repeated calls, but that doesn't mean that they can stop people from calling a business for the reason of playing a joke on them. There's no concrete way to accurately decide what someone's intentions are in this situation.