Voice mail on a cell phone works similarly to a message machine for a land line, as callers are able to leave voice messages in an in-box for the recipient to listen to at a later time.
-Hello. I’m sorry I didn’t answer your call. I’m just waiting for more important people to call. If I hear your message and deem you worthy of the title “important,” I will think about calling you back, but for now. Bye! -Beep-

For example, a message like, “Hi, this is Jim. You know what to do.” Well, not everyone is going to know what to do—i.e. they won’t leave you inclusive information. Additionally, they may not leave you a message at all. Aside from this, humor can again be detrimental to your message, this time lending itself to ambiguity and costing you clarity. For example, if a message read, “Hi this is Jim, sorry I can’t answer your call right now. Please leave your name and number and I’ll call you as soon as I can, but you already knew that right? Do I need to tell you what to do?” This is worse than the previous example as this is confusing and can also come across as rude and unprofessional. Complicating a greeting with phrasing like this is sure to cause some harm. e. Ignoring Personality & Identity: Don’t use computer generated greetings. Some users leave default messages (‘you’ve reached the voicemail box of 777-777-777, please leave a message). Believe it or not, even this can cause problems. Callers may be unsure if the voicemail box belongs to you; therefore, they don’t leave a message. Also, some may even be uncomfortable leaving information through a message in a nameless voice message box. As such, impersonalization can cause ambiguity, which again can hurt the effectiveness of your voice message system. This doesn’t mean you have to make an elaborate greeting if you don’t want to, just insert your voice and name so at least callers know they’re calling the right person.
Obviously, I need to update it. And if you haven't changed your voicemail greeting in over a year, you're likely in the same boat.