Disputes Cell phone contract/unfair billing

I signed a 2 year contract with Mtn, everything went well and towards the end of the contract a month before i informed Mtn that I am not upgrading and they told me that i must pay outstanding of R650, i DID THAT and I have proof regarding that. Later on I received sms that I owe the R4200, i phoned them and wrote emails to request explanation for that, I haven't received any feed back from Mtn only sms that threatened me from their legal department. I consider that this term is unfair pursuant to the [Consumer Rights Act 2015 OR Unfair Terms in Consumer Contracts Regulations 1999] for the following reasons: [set out your basis for challenging the terms] For the reasons stated above, this term is not enforceable against me. I need a good explanation what the bill is for, because I don't owe them anything and they are not entitled to additional sums that they're claiming. How can I be charged for their glitch!!! I have enough money in my account for the original debt. It is not my fault that they are hungry of knocking people. I therefore request Mtn to withdraw charges against me and to remove me from ITC list, because I don't owe them anything. This level of service is pathetic.<br>