Cell C debits my account when they feel like putting through debit order.

I've phoned Cell C about 5 times about debiting my account before my normal debit date. I keep on telling them that my employer pays salaries on the 25th of the month, even if it is a public holiday. One exception is in December when the pay my salary on the 23rd, NOT A WEEK BEFORE THE 25th. They flatly ignore me and debit my account before the 25th. This month they debited my account on the 22nd. Then I receive a SMS stating there was no funds in my account. What in the world gives them the right to debit my account on a different date as the date I signed my debit order for. They are F#cking around with my credit score. I am soooooooooooooo sick and tired of Cell C deciding on their own, without my permission, to debit my bank account on dates as they feel fits them. I phone them when I recieve the sms saying they will debit before the time, but I am wasting my time, obviously, because the still do what they want. I just wonder if other cellphone companies do the same. WAKE UP CELL C, NOT ALL EMPLOYERS PAY SALARIES BEFORE THE TIME WHEN THERE ARE PUBLIC HOLIDAYS.