Just as your right to wave your fist stops at my nose, your right to chat while driving stops at my bumper.
That's all well and good except that what you are advocating for is a law that prevents me from talking on the phone in my car regardless of my proximity to your bumper. Perhaps a better solution (if there just has to be one) would be to impose a penalty if it can be proved that cell phone use contributed to a collision.
There are any number of things that cause inattention to driving... why stop at cell phone use? Why not include talking with a passenger or eating food or listening to the radio? This sounds like a slippery slope.