The basic rule of thumb is that callers should hear one of two things when they first connect with your voicemail — either an apology for not being able to answer the call or a “thank you” for having called. You can do both if you prefer, but keep it short and to the point.
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.

Need script options? Take a look at these templates (and modify them as you please):
1.) Herzlich willkommen bei der Mustermann GmbH. Leider ist das Büro derzeit nicht besetzt oder Sie rufen ausserhalb der Geschäftszeiten an. Sie können aber gerne eine Nachricht hinterlassen, oder eine Email an [email protected] schreiben. Vielen Dank für Ihren Anruf.