Why can’t Don call the professor himself?

M: Math Department. Dr. Webster speaking.
W: Hello, Professor Webster. This is Janet Hill calling. I live two doors down from your teaching assistant, Don Williams.(8)Don asked me to call you because he’s lost his voice and can’t talk to you himself.
M: Lost his voice? That’s too bad. Is there anything I can do for him?
W: Well, he has a class this afternoon from two-thirty to four, and he won’t be able to make it. But he doesn’t want to cancel it either.
M: Does he want me to try to find somebody else to teach it?
W: No, not exactly.(9)What he wants to do is get someone to go in for him—just to pass back the midterm exams. He’s already marked them and they are on the desk in his office. The whole thing wouldn’t take more than...ehh, ten minutes or so.
M: Two-thirty, you say? Well, I’m free then, and I was going to be on campus anyway, so I could do it for him. Where’s his class?
W: Carter Hall, room two-fourteen. Will you need his office key to get the exams? I could bring it to you.
M: Well, actually, that won’t be necessary.(10)We have a master key in the Math Department. So I can get into his office.
W: Thank you. Professor Webster.
M: My pleasure.