What's the best prank call you have done?

Question

What's the best prank call you have done?

Answer 1

While living in a dorm during college, we called a girl, who we really did not care for, and told her we were maintenance.  

We told her we had a report that the toilets were not working on her floor.  We asked her if she could flush them all (4 restrooms/4 toilets each) to check to see if they were all working properly.  After about 10 minutes or so, she returned to the phone, huffing and puffing, to say they were all OK.