We should be able to make calls on our cellular devices if we need to. A case of "need to" would be if something at home were to happen and your mom/dad needed to notify you. A case of "don't need to answer but I'll do it anyway" is if your best friend is calling/Face Timing you from Math and you answer.
Schools have the right idea by restricting phone use. It also saves the kids some money since it also saves them data (or should save them data.)