Before you leave a voice mail message for a person who could possibly hire you for a job or internship, make sure you know what you're going to say. As ridiculous as it may sound, rehearse saying the message out loud. Do it often enough so it doesn't sound like you're reading it. You'd be surprised how easy it is to start rambling like an idiot and continually repeat yourself. Something I heard recently that I thought was a good suggestion was to call your own cell phone (even if from your cell phone) and leave a message when you're driving or just have some down time. Once you listen to it, you'll be able to pick-up on any annoying ticks (um, you know, speak at lightning speed, etc.) that you have and correct them.
Another minor little detail to consider is what you'll say if the person actually answers. I've been in the office late at night and answered the phone to a startled person on the other end. Don't discount the possibility of someone being at his desk. Additionally, sometimes the best time to reach the person you're trying to speak with is in the morning or the evening, before or after traditional work hours (7:00a-9:00a or 5:30p-8:00p). I know that I am NOT a morning person and was often in the office late at night. During the day when I'm running between meetings, I might not have time to speak at length with someone, but later in the day, I usually do.
Something I forgot to put in the last post about the message you have on your voice mail intro--be sure to say your first and last name if you have a name that people often butcher. None of us want to intentionally butcher another's name. This will help make sure they at least get close to pronouncing your name correctly when they do speak with you.