In my humble - but undoubtedly correct - opinion, the cell phone should not be used when:
You're in traffic. Exception: You're on a deserted highway in Texas where you can see 100 miles in either direction.
You are in a restaurant. Most people don't want their meal to be accompanied by a nearby imitation of Foghorn Leghorn.
You're in a theater. A few weeks on a chain gang would be suitable for this violation.