Cancelation of contract account

I had a contract with MTN which has been settled. I have contacted their retention department for cancelation and I was provided with a reference number. To my surprise, today this morning I received a debit notification for payments for the same account which has been settled in September. I am frustrated because their 135 number is not helping at all and they seem to be enjoying what they are doing. I have been their regular client for years without skipping payment and this is how they thank me.