I really don't understand the argument that people can't concentrate on driving and hold a conversation at the same time - surely driving, being immediate (and visual), takes priority.
I frequently hold very long conversations in the car on my (car kit) phone, and if it's not someone I know well, I tell them that I'm driving at the start of the conversation. Then if anything "exciting" happens (i.e. approaching a 'knot' of heavy traffic, hgvs etc on a previously clear motorway), I say "hang on a sec", and go quiet while i negotiate the hazard. If they don't like it, they can call back another time. I've never had any complaints... How difficult is this ?
On the subject of car kits, having used both handsfree kits and the "clamp phone between shoulder and head approach" (much more difficult to detect as the phone can't be seen and it just drops into your lap if you straighten up

), I would
never use a phone for any length of time without a proper hands free kit now - physically having to manage the phone, check you're not about to get pulled over etc definitely interferes with safe driving.