Telkom trying to **** me

I have a contract with Telkom. I changed my account number, I called Telkom to inform them of my new banking details and I was informed that the account needs to be open for 3 months for them to deduct. I started making cash payments. Every month on the 20 I make a payment and then on the 2nd month of making payments, they started debit order rejection. I ignored that because the payment was made on the 20th as per agreement and I reported to them that the account is closed immediately after closing the account. In December I paid on the 18th but I was charged an amount for debit order rejection. According to the invoice I owe them 900. I called yesterday and I was told that my account cannot be changed from debit order to cash payment and I should have paid for my contract 10 days before my debit order date. I'm so disappointed. I'm scared to update my banking details now because they will deduct 900 from my account and it's not my fault that they refuse to update my account because it wasn't active for 3 months.