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.
Record and play the message while the caller is waiting to be connected. An excellent way to professionally introduce your business to the callers and set it apart from the competitors.

1.) En estos momentos todas nuestras líneas están ocupadas. Por favor, tenga un poco de paciencia, en seguida le atenderemos -PAUSA- ¿Ha visitado ya nuestra página web www.johndoe.com? Sus preocupaciones nos importan, por favor, manténgase a la espera, le transferiremos lo antes posible. -PAUSA- Un momento por favor. El próximo miembro del equipo que esté disponible le atenderá en breve.
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.