Payments deducted before arranged date

At the Greenacres Telkom Mobile store I was informed that the debit order will go off on the last day of the month. I informed them that I get paid on the last day of the month irrespective of which day it falls on. Telkom has been deducting the money from my account before the last day of the month and the bank is charging me R150.00 each time. I contacted the call centre and arrangements was made for the 5th of each month starting from the 5th November but the debit order went off on the 25th of October. I am a pensioner and cannot afforrd all these unnecessary deductions from my account. Please advice what more can I do to make Telkom listen to the customer.