At present, a driver talking on a handheld phone while going down Highway 91 would be on the wrong side of the law while passing through Idaho Falls and Pocatello (and, soon perhaps, Blackfoot as well), but on the right side of the law elsewhere. Phone your mom while driving in Butte County, no problem. Cross into Blaine County, and you’ve broken the law.
The legal patchwork that has developed across the state should be resolved by a state law banning driving while talking on the phone. A driver shouldn’t have to research the laws of every municipality they pass through in order to stay in compliance with the law.