People who call and either don't leave a message, or worse leave a pointless message - don't tell you what they want.
Had one earlier today. Didn't know the number. Listened to the message - a company that I have no current or outstanding business with, asking me to call them back. Why? WTF do you want?
Obviously a sales call. I won't be calling them back. Can't tell me what you want on the initial call, I can't be bothered to call you back.