Unhappy customer

Iam so not happy with mtn ,you take a contract with them and you are told you will be paying this much and you reach an agreement with them come pay date you end up paying more than what you were told you will be paying ,and everytime you try and ask how did I get to this amount the same person who was helping you with the contract you took has no idea why you paying more than you were told. And they will give you a number to call ,and those people don't give you a straight answer . I have never missed any payment on my contract with mtn but right now ,now that my contract is ending with them February month end they telling me I owe them R3000 for what exactly because I have never missed a payment .they suspend my simcard I cannot make calls ,I cannot use data same simcard that gets me money they suspended it ,mind you my work requires data .