49 CFR §392.80

1. (a)Prohibition. No driver shall engage in texting while driving.
2. (b)Motor carriers. No motor carrier shall allow or require its drivers to engage in texting while driving.
3. (c)Definition. For the purpose of this section only, driving means operating a commercial motor vehicle, with the motor running, including while temporarily stationary because of traffic, a traffic control device, or other momentary delays. Driving does not include operating a commercial motor vehicle with or without the motor running when the driver moved the vehicle to the side of, or off, a highway, as defined in 49 CFR 390.5, and halted in a location where the vehicle can safely remain stationary.
4. (d)Emergency exception. Texting while driving is permissible by drivers of a commercial motor vehicle when necessary to communicate with law enforcement officials or other emergency services.