MTN ripping us off

MTN decided to change their billing dates as a strategy to boost their finances. I have an existing contract for R322 and this month they decided to take R511 citing the new billing system. The contract I signed is fixed and isn’t supposed to be impacted by their internal changes. I want the R511 reversed and my normal fee of R322 deducted. After this I even want to port my number to a different service provider. This is broad ****************, my cell number is ***. Please make sure the reversal is done ASAP