Hi, you've reached f! I'm around, just avoiding someone I don't like. Leave a message after the tone, and if I don't call back -- it's you!
[annoyingly cheerful] Hi! f's answering machine is broken. This is the refrigerator. After the tone, please speak very slowly, and I will stick your message to myself with these little magnets.
You know what to do.
I'm sorry, Dave. I'm afraid I can't answer the phone right now.
This is a telepathic thought-recording device. After the tone, please think about your name, number, and reason for calling. Then I'll think about returning your call.
Hello?... Halló?.... Алло? もしもし? Well, if you're not going to say anything, at least leave a message.
[slowed-down recording] HI, THIS IS DEATH. I'M BUSY RIGHT NOW BUT IF YOU LEAVE A MESSAGE I'LL BE RIGHT WITH YOU.
[computer-distorted voice] FOXTROT-LIMA-ALPHA-SIERRA-HOTEL. KEY-IN-DESTRUCT-SEQUENCE-NOW. THIS-TERMINAL-ALSO-ACCEPTS-VOICE-MESSAGES.
[thick Russian accent] Thank you for calling the KGB headquarters. After the tone, please leave a brief description of the secret you wish to sell. Don't worry about leaving your number. We'll find it.
Other, far more awesome message which I will share in comments.