Mark: Is that you, John?

John: Yes, speaking.

Mark: Tell Mary we'll be late for dinner this evening.

John: I'm afraid I don't understand.

Mark: Hasn't Mary told you? She invited Charlotte and me to dinner this evening. I said I would be at your house at six o'clock, but the boss wants me to do some extra work. I'll have to stay at the office. I don't know when I'll finish. 
Oh, and by the way, my wife wants to know if Mary needs any help.

John: I don't know what you're talking about.

Mark: That is John , the engineer, aren't you?

John: That's right.

Mark: You work for the Overseas Engineering Company, don't you?

John: No, I don't. I'm John, the telephone engineer. And I'm repairing your telephone line.