Debit Order issues

I cancelled my contract with Cell C in the beginning of July. Since then the debit orders has been going off 3 times a month and no one at Cell C can explain why that happens. I stopped the debit order with the bank and I sent a letter to Cell C to stop the debit order on my account. No one at Cell C seems to be keen to help. They always call when I am not available at work and when they leave a message on my voice mail, no one can hear a word the operator is saying. THE WORST SERVICE I EVER HAD!!!!