Poor billing system, *********** / unskilled staff not willing to resolve

My favorite place to shop is truworths, im and online shopper and enjoy the experience. However my problems stem from their billing system bot updating and calculating correctly I have a 6 month account and if I owed R6000 my installment would be R1000 pm. Zain in the accounts call centre confirmed their system is incorrect and cant update due to arrears off September premium. The supervisor off the accounts team Mustapha was too busy to take a call, I need help to rectify the balance due