They called my son's cell number and I answered it. Said they wanted to sell me adult products. I told them to add the name to their do not call list. Caller interrupted me and just kept trying to sell the product.
Contact the cell provider to get the number blocked. Or put the phone number in his contacts list and assign it the ring tone "no ring". It won't stop the calls but it will keep the phone from ringing while he is in class.
Check his messages often and delete the obscene ones. Having porn or near porn on a phone in school can get your son in a lot of trouble.