EARLY DEBITS WITHOUT CONSENT

I have a debit order with telkom that goes through on the 1st of every month i then manage my account every month to make sure there is money on that date. They however debited on the 27th of the previous month, earlier than the agreed date without notification which caused insufficient funds charges on my account as i only transferred their amount to my account on the evening of the 28th what an inconvenience and poor service .What now telkom please contact me to resolve this because i would like to pay and not be double debited next month