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.
Context does matter here. If the caller would reasonably expect you to answer the phone (such as if they were calling a store or other place of business), an apology for not being able to get to the phone makes the most sense.

You may think your voicemail message is professional. But when you listen back it could sound rushed or shaky. Listen to it regularly to see if changes are needed.
4. Funny Voicemail Greetings. Hi, you’ve reached [your name] at [X company]. We are busy trying to save the world by [what your company does best]. If you want to learn more about how we do it, please leave us your name and phone number, and we will get back to you as soon as our mission is complete— which should be fairly soon.