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.
A relatively unprofessional one — like mine, for instance — does the opposite: It encourages prospects, recruiters, and potential connections to run in the other direction.

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.
On occasion the option to send a call straight to voicemail is not available. If it is determined that the recipient does have a voicemail box but the Voice Mail option does not appear as in the screenshot above, this is likely because the recipient is a Skype for Business on Mac user.