As a teacher, there are two sides involved here. No, there is no reason a student HAS to have a phone in the room. It is nice for them when they do little things like I had students today use them for (create a drawing of how to represent something) so they had a model to look at while they drew. At the same time, I have a lot of students get extremely belligerent and upset when I tell them to put their phones away during class because "I"m just texting my parents. Stop being a ****."
The easiest answer is to just not let them out at all since it isn't a necessity, but parents have complained that they have to call the office to get ahold of their kid as well. Both parents and kids don't realize how disruptive it is to have a student in the room being defiant because their parent called.