IIRC, there was considerable discussion on the level of attention required of the driver between full and half duplex.
Personally, I think appropriate methods of cell and radio wireless should be allowed. An endorsement on your DL perhaps, but technology can make this do-able. The interface is poor, and not enough adoption of headsets by drivers. I still like the better wired ones as well as Bluetooth.
What if cellphones quit working over 10 mph or so unless a proper headset was attached, measured by GPS, and when located on a road?