A prank call is a prank call. Except when it isn't. Phone calls can often be quite tricky things. Because the person on the other end of the line can't see you, it is often necessary to explain who you are and in which situation you are right now. And because of the same reason, the person at the other end has to take your word for it. Well, the persons called will simply refuse to believe you. They will think that you are just a prankster with a sick/unfunny sense of humour. They will either hang up without any further words, or will angrily snark at you, and then hang up.