Problem, as I see it: if I were really to do the job properly re mobile phones, I'd be 'shopping' just about every fourth or fifth motorist I pass on the road. I don't have the energy, and I think the police would be overwhelmed and have all the more reason not to follow any of them up.
Now, if the initiative were to come from the police, say an announcement of some sort of 'clampdown' week, encouraging fellow citizens to grass up on offenders, and if the police diverted extra manpower to deal with the expected influx, that would be different. Of course the regular offenders would be wise to it too...