Incorrect contract amount after apple store trade in upgrade

Hi Mtn,

I have two contracts with mtn, and the issue I’m having is with my iPhone contract. I have upgraded in the fourways Istore last year April 2021. I used my previous IPHONE as a trade in. The new contract amount should be: R849.01. I had no issues with my account until October, the contract price went back to the original amount of R1100. I have went into the mtn stores, they unhelpful. I called the call center every week. I have so many requests which all was closed by this team or that team or they said it’s the wrong request. My bill is still not fixed what do I do now? Every month my account gets blocked and no one helps. Kindly advise: my number +***