The prank phonecall genre, 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 being good-natured are funny, and it's fine to laugh at someone who has to be nice to you whatever you say, because it's their job.” That, to me, is not funny.
It's just cruelty.
Now don't get me wrong, a lot of humour is cruel. I even agree with Stranger in a Strange Land (Sorry about the spoiler, Danna) that cruelty is the essence of humour1. To me, however, cruelty can only be 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 to the rule, 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. Even outside sitcoms: the interview victims of Norman Gunston or Ali G invite ridicule through the hubris of their celebrity. The audience of Dame Edna knows that it's risking her barbs by buying a ticket.