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.
@Fa5t3r - I would try a wide range of different tones when recording and play them back to see what goes over the best and what projects the image you want people to hear. Remember that some people may be going to hear this message over and over again depending on how often they have to call you so you want it to be very pleasant and not at all irritating.

It’s a good practice for each of your team members to have their own personal business phone numbers. There are many reasons why they shouldn’t use their personal cell phone number for business, so you’ll want to give them their own phone number through your VoIP provider or phone system.
You should acknowledge that right off the bat by apologizing that they couldn't reach you directly.