Or write a email to them. Simple but sometimes useful.

It is never unprofessional to make this call. Be dedicated to your career and take every chance to know your drawbacks and improve it is a very professional behavior.

Try to do it in this way,
1. Find out the interviewer's namecard and give him a PERSONAL call instead of a business call; take him as a mentor in your career rather than interviewer. Never call the number in the reject letter or found on the company website. (If you don't have his name card, give up the idea of calling and ask for one next time.)
2. Mention something special in your interview so that the interviewer can recall whom you are. (If you really can not find one, don't call.)
3. Organize your words so that it gives people a impression of you are looking for points to improve yourself, instead of challenging their decision.
4. Write a email to show your appreciation after the call.

If you are a fresh graudate and already rejected. You really got nothing more to lose with this company. If you called, the interviewer may tell you frankly your significant problem which you never ever pay attention to. So don't be afraid of those who thinks you are unprofessional and lose a precious chance to improve yourself.