Incorrect Billing for Contract and not willing to resolve this

I'm very disappointed at MTN. I took a contract with them in November 2018. The installment was meant to be R199 per month however I am being billed R299. I have tried to resolve this however it appears that nobody is willing to assist me. I have tried to get the contract from the Cavendish store however nobody replies to my email. The only proof which I have is the contract summary which clearly states R199 however they stating this is not enough.