Your voicemail should be very specific. It should be short and urgent. Use word's like "need", "should", "must" , or "have to" to create a sense of urgency. For example, say something like, "We need to discuss..." or "We should talk about..."
My 12 y/o niece put on her cell phone message, "Leave your name and number. If I love you, I'll call you back." Although not earth-shatteringly funny, it was cute the way she said it. In college my roomate was a cop, he told me that the worst things you can say on an answering machine were 1) telling them your name, 2) telling the caller your number, 3) and that you aren't at home. Well, here's what I did... Thats probably good advice for a home phone, but at the same time, I don't think it applies to a cell phone really. I used to have a longer message aimed at keeping people from leaving messages ... but I hate getting voicemails for the most part. One summer I had my little cousin start singing take me out to the ball game, I cut him off after a few lines, but it was just my version of satire against parents who have their kids sing, and it was pretty funny.

“Hello, this is Morgan Freeman. I wish I could tell you that Morgan Freeman was available to take your call. I wish I could tell you that, but this is no fairytale world. Morgan Freeman is gone now; to where I cannot say. But if you’re lucky, I might just call ya’ back. Get busy leavin’ your message.”
You have reached the office of [Business Name]. All of our representatives are currently busy. However, if you choose to do so, you may leave a brief message, including your name, number, or email, and we’ll be sure to get back to you as soon as possible. Thank you for calling [Business Name].