Payin more than the agreed amount

Mtn on thier contracts they keep on charging clients extra interets on fixed packages even when you want to cancel they are refusing and charging you still My fixed contract packager was R570 pm

But not they are charging me R1140 per month , when i call their consultants are not willing to assist, i went to the branch at campus square they said they cannot assist with anything i should call MTN😭😭😭😭😭