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.
It's easy to get carried away in a voicemail and include more detail than is necessary. We've all been in that situation where the automated or pre-recorded voice on the end of the line goes on and on with more information than you can take in.

You can also upload an unavailable greeting using the OnSIP app. After logging in, click on the Voicemail Settings gear icon at the far right. Make sure that the Custom Greeting is toggled to "on." You will then see an option to upload a .wav file of your choice. This recording will serve as your unavailable message.
A business voicemail greeting is more important than you think it is. While many users pine away at intuitive user menus and expansive feature sets, many forget the importance of their recorded voicemail message. While this may not sound like a travesty, it could very well could be.