Here's another call to the prankee where the prankster is trying to save a life. By making the prankee think that there's ransom involved, the prankster really scares the heck out of him/her.
You: Okay mister. I have the money you asked for. Now where should I get it?
He: What? Who is this? What money? What are you talking about?
You: Don't play games with me. Your ransom. I have the money you wanted.
He: I think there's some misunderstanding. You've got the wrong guy.
You: I just want my son back. Please take the money. Give me my son back.
He: Sir, I really think there's been a mistake. I don't have your son.
You: What? What did you do to him? I told you I'll get the money. You didn't have to kill him.
He: I haven't killed anyone.
You: Oh God, my son... my son. You killed him. I WILL FIND YOU. You will regret this.