It is, of course, quite possible that cell phones have led to a larger increase in the accident rate, but not the death rate, for any number of easily imaginable reasons, including that they may primarily be used in city traffic, which is relatively low speed. But it would be helpful to have some actual data.

There is interesting data on red light cameras, which shows an increase in accidents, but a decrease in fatalities...the accidents being caused by people slamming on the brakes when they realize there is a camera at the light, resulting in a relatively safe rear-ender. The decrease in fatalities being caused by fewer cases of people running red lights, which is more dangerous because it tends to happen at higher speed.

Of course, even low speed collisions are dangerous to pedestrians and bikers.