I used to leave long messages. For example:
"What will be your first cybernetic enhancement? Mine will be a spinny nose. Because many of my friends will get laser eyes, and with their laser eyes, and my spinny nose, together we can play CDs. Leave your name, number, and first cybernetic enhancement after the beep."
The joy was the responses I would get. Some people would play along and tell me about their cybernetic enhancements. Some people would be laughing too hard to leave a coherent message. My father would bitch and bitch about having to sit through the entire message each time he called (which invariably cracked me up -- he was so whiny about it that I never did explain how you can bypass a voicemail message by hitting the pound key). And then there was my favorite message: a guy who was probably three sheets to the wind, yelling, "Sonja?.....Sonja?.....What the HELL was that, Sonja?"
So that's my suggestion: leave a meandering message that asks people a strange question. Ask them their preferred manner of death; what dangers the seven seas hold; to leave tehir message in rhyme; and so forth.
Or you can do as my mathematician friend did, and leave the following message in a pleasantly neutral voice:
"You have reached an imaginary number. Please rotate your telephone ninety degrees and call again."
Daniel