Yes. Although not every state currently bans use of handheld cell phones while driving, the clear trend favors such restrictions. Every year, more states ban texting while driving, require drivers to use hands-free devices if they want to talk on the phone, and restrict certain drivers (such as teens) from using cell phones altogether. As an employer, you have more than driver and pedestrian safety to worry about: Your company could also be legally liable if an employee causes an accident while talking on the phone. For information on employer liability, as well as tips on what to include in a cell phone policy, see Nolos article Cell Phone Policies for Employees Who Drive.
For legal advice, you'll need to talk to a lawyer.