Like prank calling? You'll love this.

[Sidoh]

http://www.google.com/help/faq_clicktocall.html

I'm not so sure this is a good idea...

[Killer360]

Awesome! 

[AntiVirus]

I wonder how that's going to turn out...

[Armin]

Not sure if they just added this, but:

#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.

[Sidoh]

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.