Consider for a moment how your phone is currently being answered. Professional courtesy is quite often not the standard for many college students. An abrupt "Yeah!" could be listed among the more courteous greetings. The more outrageous remarks will often buy you a major black mark in the professionalism category—even if it was your roommate answering your phone and acting crazy. A simple "This is ____" is always a pleasant change for a college student call. Make the change today, before the next (or first) employer phone call. If you have a landline, you should also encourage your roommate to do the same.
Even in today's fast-paced world, customers don't like being greeted with an automated message. According to the New York Times, callers of a certain age are put off entirely if they are answered by a voicemail instead of a real person.

This is optional, but you can also leave another form of contact on your voicemail, like an email, that the employer who is calling can utilize. This will show that you want to get back to them as quickly as possible, and you are giving them multiple ways to do so. This isn’t necessary, but can be a good step if you want to squeeze a little more into the voicemail without going overboard.
When you start to record the business voicemail greetings, be sure to pay attention that the professional voicemail is not able to pick background noises that could make your message sound distort and inaudible.