the question is over how much of a contribution the phone is (especially compared to other distractions) and if it's really a "significiant" number of accidents.
I don't think that anyone disputes that some people drive significantly worse while talking on the phone, or that this results in some accidents.
The dispute comes when you assume that banning phones will significantly reduce accidents.
So far it appears that this isn't the case.
P.S. A law against something bad that everyone ignores is worse than not having a law against that same something, it encourages people to think of the law as something that doesn't really mean much, and it leads to people looking at cases where laws are enforced with the slant of "why were they really out to get that person"