The best after hours greetings are brief, direct, and punctual. Do not waste the customer's time and give them a reason to hang up by adding in unnecessary details. The information in your after hours greeting should remain concise and essential to the caller's needs.
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.

“Gracias por llamar al escritorio de (ingrese su nombre) al (ingrese el nombre de su compañía). Estoy en una llamada o lejos de mi escritorio, pero si deja su nombre, número de teléfono y el motivo de la llamada, le devolveré la llamada lo antes posible. Gracias.”
Now this is set up I won't feel bad about summarily ignoring voicemail letting agents leave me ;)