Billing Error

My installment for December failed, I setup a payment arrangement my plan is for R625 I paid R50 on the 24th Jan which lowered the balance to R575, I set a payment arrangement for over 8 months and was given an installment of R71.87.

The moment I accepted the agreement it changed to 1150 and installment was R137. So I thot maybe they have included the installment that was going to be due in February.

Feb installment deducted. But balance is still the same.

I spoke to their billing team and was told the matter would be escalated its been over a week still no one has contacted me.

Instead theres attempts made to deduct the 137 installment and I want to know how my balance doubled yet I owe only one installment