Irregular subscription charge by MTN

I have a 24months contract with MTN which I renewed in March 2023. I'm a loyal customer to MTN for more than 12years on contract. This year I've upgraded by adding another number for my son after completed his Matric and my subscription is R814 pm which is a Standard contract meaning I cannot upgrade or downgrade during the duration of my contract. In May I was debited R2717 instead of R814 and when I queried it I was told that he had upgraded and used more Data than he was supposed to use.They further said my son requested the upgrade but my son admitted that indeed Data was made available for him and he thought that it was a compliment since it was a new phone and he insist that he never requested it. I made calls to MTN several times including emails but all my efforts were in vein.me.I have lodged the complaint to the Ombuds but their responses are totally unacceptable. Surprisingly both ICASA and CGSO Complaints Ombuds refused to assist in this matter saying it not their jurisdiction. I hope someone will listen to me and understand that MTN false upgraded the contract without my consent as the owner of the account. I need to be reimbursed my payment.