Shocking accounting service

I changed my debit date with MTN on the 6th December due to my salary date changing. I have called them and checked several times and been told that the date has definately been changed on their system yet they contintue to debit on the old date - without funds the debit bounces and i get rude emails from them threatining to disconnect me. During February i check again and was agains assurred that the debit would only go off on the 28th, but it went off on the 25th fortunately with the 28th being a Sunday i was paid on the friday or it would have also bounced - i have called in, written in - to which i get an email confirming an 8hour turn around - yet still nothing.