If the previous answers were not enough, another aspect to consider is health hazards and the ethical issue of the privacy of self and others.
Morally speaking, it would not be correct for a person to make a strange audience participant in their private conversations. Therefore, opening a dialogue on the phone in a place where other people can hear you evades the privacy of the talker, and the rights of others are violated by having to be around a talker.
Ethically, there is a supposed danger connected to cell phones: First, radiation may cause brain tumors, although this has not been openly proven. Second, it may make you lose control of the vehicle. So,the most ethical thing to do is to make the right choice of leaving cell phone use to emergencies, or privately without disturbing others.