Harassing me over paid-up-letter

I had an account with Ackermans I have been trying pay off January i got a call that I should make a payment because 01 January Debit did not go through I understood an the lady ask me when can I make the payment and for how much can I afford?

I asked for the settlement amount she said Four hundred and something... amd the we ;arranged for once off debit Order as it was the last payment. Then a week after I Called Ackermans for ''paid up letter'' they tell me that I still owe them THE AMOUNT i WAS GIVEN WAS WRONG when I ask for clarity the say the matter has been escaleted to managers or supervisors they will update me within 7 days and now no response but they call me everyday harassing me about next payment