The prank phone-call, at its essence, relies on the generosity of the person on the other end of the line. It preys on someone's willingness to do their job to the best of their ability, or it preys on the victim's willingness to give the perpetrator the benefit of the doubt. So in essence, what it's trying to say is “People who take strangers at face value are funny, and it's fine to laugh at someone whose job it is to be nice to you whatever you say.” That, to me, is not funny.
It's just cruelty.
Now don't get me wrong, a lot of, even maybe most1 humour is cruel. But I can only find cruelty funny if the victim is deserving or complicit, when they bring it upon themselves. That's why the best sitcoms are populated by flawed people2, because really good comedy is about bad things happening to people, and bad things happening to nice people aren't nearly as funny as people who bring misfortune on themselves.
It's why a good scam is entertaining—all good scams target the greed of their victims. But prank phone-calls? No thankyou.
1 Puns are the exception, although you could argue that puns are cruel, just to the listener.
2 Basil and Cybil Fawlty were cruel. Manuel was incompetent. Polly was nice, notice how she escaped most episodes largely unscathed? Lister, Rimmer, Cat, Kryten and Holly. The vanity and selfishness of the cast of Seinfeld. Each Simpsons character, defined by a character flaw.