MTN Incorrect invoicing

As a vat registered company it is imperative that we receive all invoices and that all invoices are correct. We have 14 cell phone contracts with MTN. For 3 months MTN has been sending us incorrect invoices however the amount which was debited from our bank account seemed to me in the correct range. I have been contacting '***' numerous times and i have only received an answer for 1 month's incorrect invoice - and no credit note yet. The incorrect invoicing has happened in a previous year as well and i am fed up because there is just no response from MTN!! Very unhappy customer.