I wish they would leave a message even if they just say something like "This is a call from the University of S Florida (Alumni Association). We will try you again at a later time."
If would be very helpful if a voice mail was left indicating who they were and why they are calling. I am one of those individuals that may not answer a call from a phone number not in my address book - to avoid solicitations and crank calls. No message left implies that is a telemarketer or crank call.