If you have received serious threats to yourself or your child on the mobile you should contact the local police immediately. Remember to keep a record of any text or voice messages as evidence.
If your child is being bullied or harassed by someone on their mobile, contact your mobile phone provider who will have abuse teams who can help you in blocking nuisance or malicious callers, they can sometimes arrange for your child to have a new mobile number. Obviously if the bully is someone your child knows from school, you should approach the school to deal with it. Every school should have an anti bullying policy and should be able to help you.
If you or your child are receiving premium rate text messages on your phone, asking you to call 09 numbers, then you can report this to PhonepayPlus—the Independent Committee for the Supervision of Standards of Telephone Information Services. You can contact them on 0800 500 212 or visit their website.