"No person, when operating a motor vehicle, shall permit to be on or in the vehicle or on or about his person anything which may interfere with or impede the proper operation of the vehicle or any equipment by which the vehicle is operated or controlled, except that a person may operate a motor vehicle while using a citizen's band radio or mobile telephone as long as one hand remains on the steering wheel at all times."
I suppose one could argue whether "mobile telephone" means a radio telephone (think the TV show "Canon") or cellular telephone. Also, "citizen's band" does not mean to confine the radio to the 11 meter band, but would include other Ham (amateur) bands.
Background to History:
Back when cellphones began to proliferate, a lot of our guys back-east were writing these people for 90-13. Since the original cell phone users were more "upscale" than todays clientele, complaints were made and the "mobile telephone" exemption was added to 90-13.