MTN is useless

I am tired of MTN and their customer service call center, I have a contract with MTN that I recently upgraded that I pay cash every month and the reason for cash payments was cause of wrong amounts being debited from my account, so this month I paid and my account has been cut off cause according to their system I owe which is wrong, I have since taken the laptop back to the store I got it at and told them to call me when I can use it but still nothing and month end I should pay why cause I cant use the Laptop when I need it most. MTN is useless and the staff is not helpful.