Early debit orders

I bought a phone from cell c over 24 months.the phone is wonderfully and the debit orders smooth.then suddenly a unilateral notice that the debit orders will go through on the 27th december despite the fact. That I indicated the end of the month.i will not have money in my account to cover this.who will pay the costs of a dishonored debit order.my phone number is ***my name is Isobel Duvenage.