Unexplained additional amount charged

My Dad has three contracts with MTN which I pay for and I pay before the due date every month. For the past two months we have been charged an additional R126 which the consultants at the store can’t even explain what is it for. We are unable to get through their contact centre as well as they need to provide us with an explanation as to what is the additional amount for. We even pay more than the due amount every month.