Using cellphones is a necessity of life in the business world. But employers need to strongly discourage employees from using cellphones while driving - because employers can be liable for road accidents caused by worker cell phone use.
Smith Barney paid a $500,000 settlement to the family of a motorcyclist killed by one of its employees making a work-related call after hours on his own personal cell phone.
What's the solution?
As usual, prevention is the major part of the cure. Your handbook should have a policy stating that using cell phones will driving is against company policy. If an employee must make a call, he/she should pull off at a safe spot to make that call.
You can't stop an employee from calling while driving, but if they violate a company policy, at least you can show that you tried to prevent the behavior from occurring, and this may help you if a legal action takes place.
And, a written policy provides a basis for disciplining or terminating that employee. Otherwise, the employee can always state they didn't know such a policy existed.
Even though more states are adapting 'hands-free' driving policies, it's still a wise course of action to get that policy in writing.