Does any car maker still build the good ol' handset car phones? I think Audi does, and I think I remember Volvo offers them as optional.

My personal system of choice would be a phone/radio override that puts phone calls on the car's speaker system, with a microphone of whatsoever kind. My experience from piloting is that headphones or earplugs cut off way too much of the ambiance noise, which can be an important factor in road traffic (sirens, etc). So, a headset law may be too specialized. Something against the hand use of cellphones during driving would have been sufficient.