At this point, children are raised on mobile phones. There are even apps for toddlers. For all of the possible distractions they might cause in school, there's nothing wrong with teaching kids responsible use. Sometimes parents need to contact their kids and can't count on the schools completely. Also it helps with that curfew.
The methods by which we communicate with one another are evolving and changing. Email and text messaging are increasingly common ways to communicate and children can responsibly communicate in those ways via cellular phones. Also, land lines are becoming increasingly less common, and kids need cell phones to make calls in cases of emergency.