Not charged accordingly

Am ABSOLUTELY NOT HAPPY WITH MTN!!!!i have a contract with MTN for my phone..it’s been 3months receiving statements from MTN that I have to pay the usual amounts but lately MTN is debating less amounts and when I call they keep telling me that they will deduct but nothing is ever deducted…am forever told I different story all the time..the people from the call centre tell me my account is up to date the next thing I don’t owe anything, the next am owing a whole R1800 am really not happy and this is now affecting my credit score!!! From paying R810 now I have to pay R1800 for a fault that’s made from your side!!!!!with your call centre agents that are just USELESS AND NOT HELPING WITH NOTHING