i can appreciate your wanting to answer the phone; however, the point is that the phone should not be answered in a college lecture room while the professor is teaching. not getting up and and leaving the room before answering the phone is both rude to the professor and, more importantly, rude to those who are paying a great deal of money for their own educations.
granted, this professor overreacted. that much is obvious. his reaction doesn't mean, however, that the student wasn't in the wrong, too. during the first day of most college courses, the professor likely addresses the issue of cell phones. many explicitly state that they are a nuisance and will not be tolerated. for the phone to go off is one thing. most of us have had our phones go off at inopportune times. for the student to answer it and talk while the professor is teaching--and while the students are (maybe, maybe not) learning--is quite another.
again, i'm not stating that what the professor did was not uncalled for and excessive. however, for a student to disrupt the class--to disrupt tens of other students who are paying a lot of money--is inconsiderate. it's really quite simple. you put the phone on vibrate before you enter the class, and you take the phone call outside the room. if you forget to put the phone on vibrate, you pick up the phone and don't start talking until you're out of the room. that is not too much to ask for. if it is, you deserve to be punished by the professor, albeit in a less extreme way than this professor did.