OVERBILLED NO HELP FROM MTN BILLING

I upgraded my contract in September, since then I have been billed for the old contract as well as the new one. I dont see why this would be happening if I upgraded my line, I was given a reference 1 - *** and told this was MTN policy to pay the old one which is obserd whats the point of upgrading?????I followed up to hear I will be contacted by billing then a manager and NOTHING. I am being over billed by R400 2 months in a row.