Greetings to Use
Here are some transcripts for some humorous voicemail greetings you may want to use:

This is (your name). If you are the phone company, I already sent the money. If you are my parents, please send me money. If you are my bank manager, you didn't lend me enough money. If you are my friend, you owe me money. If you are female, don't worry, I have plenty of money.

Greetings, you have reached the Sixth Sense Detective Agency. We know who you are and what you want, so at the tone, please hang up.

Hi, (your name)'s answering machine is broken. This is his refrigerator. Please speak very slowly, and I'll stick your message to myself with one of these magnets.

Thank you for calling the Hearing Test Line. Prepare for test one. Is the following tone louder in your left ear, or right ear?

Hi, I'm probably home, I'm just avoiding someone I don't like. Leave me a message, and if I don't call back, that someone is you.

Prank Messages to Leave
Most effective on those that listen via speakerphone with others around to hear. Also best to have someone else to leave the message that the recipient does not know.

Hi, this is Candy from Sex Toys. You haven't paid for the products we sent you on (enter date here) you naughty boy. Please call back and arrange payment, thankyou.