Teens today see nothing wrong with stepping up to a fast food counter or walking through a check-out line talking on their cell phone. Actually, it has become their rule of etiquette.
Part of this reason may be that they see others doing it, so that in their minds makes it all right. But, it is not a rule of etiquette! Quite the contrary.
Know that it is impolite to continue phone conversations when entering public places or while being served by waiters, cashiers, or other service people. Even in church.