MTN billing me more than agreed contract amount - Almost 3 months & no resolution

If there was a negative star, I'd select it. Absolutely appalled by the poor customer service at MTN. Since upgrading my contract at the end of March 2021, I have been billed incorrectly (amount is higher that what I signed up for). I have followed up on numerous occasions, it is 2.5 months since I raised this incorrect billing - and this has not been resolved. All I'm told is that 'your case has been referred to the billing department.'

Incorrect billing of customers is *****. It is ********* and an absolute breach of the contractual agreement.