Incorrect debit order date

My Cell C contract date has changed for some reason, I have never requested this! It’s causing problems because my salary day is the 1st but for some reason Cell C debit my account on the last day of the month ie 30/31st this is not what I agreed to. My Cell C account is now going thru as unpaid which affects my credit rating negatively! My bank account is going into a negative and I’m being charged unnecessary debit order reversal charges which also affects my credit rating! I have requested this be sorted out, but it’s been months now and nothing has changed! I have my own Cell C contract as well as a contract on my mother’s name which I pay for. This needs to be sorted out asap please!!