Account in Arrears

I got an sms from Telkom on 31 August saying that my account is in arrears with R953.86. But my account is in arrears because Telkom didn't deduct the R900 monthly debit order on 01 August. Because the debit order didn't go off my account is now in arrears, I refuse to pay extra money because some one at Telkom forgot for some odd reason to deduct my debit order for 1 month - because it always goes off every month except for last month. Account nr: ********** 42713