Failure to debit as per agreement and claim to sus

On the 17 July 2015 I have contracted with Telekom to give me a Tab and data for 24 months and they will deduct the amount due on the 15 of every month. The employee from Telekom failed to push in a correct day and a the result they did not debit as per agreement. Now they want to debit at 31st of August which is not as per our agreement. On informing them to deduct on the 15 of next month they threaten to suspend my line. The crux of this is Telekom failing to deduct on the agreed date and deduction on month end has been set to be taken as ***** and I usually relocate my money after a specific day. Should they try to deduct from my account on the last day of the month I will be charged as it will conflict with my agreement with bank. Please assist