Severus Snape

Professor Snape had started a new subject a few days ago—"Poisoning Potions"—but you hadn't understood a thing. You were too afraid to approach him and ask in person, so you asked a classmate for his number. She gave it to you reluctantly, because Snape hated anyone sharing his number without his permission.

"Good evening, Professor. I'm a seventh-year Slytherin student and I have some questions about the new topic," you wrote.

He literally took an hour to reply.
—Who gave you my number? Why don't you ask me in person? I don't like being called on the phone for class-related matters.