If you have a voice mail and want to play a prank on your friends, my idea is just for you!
The prank is easy. Its point is joyful message. You may think out something yourself, but may do the following: Record some usual message - "Hello, it's Tom Johnson, now I cannot speak to you. Leave your message after the signal". The person is already ready to say his message, but alas! Instead of usual signal, he hears a high sound (you may record say, clock's ring), lasting for about 10 seconds. After it, "deal the final blow", say something like: Speak slowly, I am not very quick to perceive the information!
When I made such prank, the only I could hear through recorded messages by voice mail was laughing ..