You’re growing tired. Your eyelids are getting heavy. You feel very sleepy now. You are gradually losing your willpower and your ability to resist suggestions. When you hear the tone you will feel helplessly compelled to leave your name, number, and a message.
iPhoneAlley details a technique to trick the iPhone into using any custom audio file as your voicemail greeting.

If you don't create call answering rules, incoming callers will be sent directly to your voice mail when you’re not available.
Finally, and most annoyingly, getting set up with On Voicefeed was pretty tedious. You have to enter basic information, just like almost any other app, which is fine. But then you’re asked to enter a security code that is sent to you by text. At this point, I’m thinking, “Alright, but this better be it.” Then, the app asked me to call a number that was automatically added to my contacts in order to activate the account. Fortunately, that was the end of it, but I was still a little peeved at how long it took to get started. I’m sure that some of those steps are necessary to tap into your voicemail, but finding a way to streamline the process would be much appreciated.