49 CFR §392.82

1. (a)
   1. (1)No driver shall use a hand-held mobile telephone while driving a CMV.
   2. (2)No motor carrier shall allow or require its drivers to use a hand-held mobile telephone while driving a CMV.
2. (b)Definitions. For the purpose of this section only, driving means operating a commercial motor vehicle on a highway, including while temporarily stationary because of traffic, a traffic control device, or other momentary delays. Driving does not include operating a commercial motor vehicle when the driver has moved the vehicle to the side of, or off, a highway and has halted in a location where the vehicle can safely remain stationary.
3. (c)Emergency exception. Using a hand-held mobile telephone is permissible by drivers of a CMV when necessary to communicate with law enforcement officials or other emergency services.