Do keep in mind that anything out of the ordinary, or too long will eventually grate on the person listening. My sister recorded a message similar to the one on the film Terminator, where the message says "Hello" and then pauses so that the person on the other end thinks that they are talking to a real person, before revealing that it was a trick.
My poor grandfather kept trying to call her and fell for the trick every time. He got so frustrated I think he almost disowned her! Well, not really, but still, you've got to remember that even if it seems clever at first, it won't seem like that 50 times down the track.