In the car, the user should make sure to let the person to whom they are speaking know just in case the call is lost. Also, keep the conversation short, for safety's sake. If you're using the telephone on an airplane or in the airport, make sure the conversation is relevant. Always remember that unless you go sit in a corner somewhere, others can hear your conversation, whether they like it or not. It's useless information.
Some places are just completely inappropriate locations for a phone call.
That includes the grocery store, movie theaters, funerals and religious services and other unmentionables. The best use of a cell phone, was when I was at a trade show. Why you have to take a phone into the bathroom is beyond me. There are guidelines. Friedman is particularly perturbed when a phone rings during one of her seminars.