Vangate mall *********** workers

I am so fed up! I took out a cellphone contract with them last month of which I am due for my first installment this month. I distinctively said and have chosen the last day of the month for billing. I also 2 weeks later received a sms to choose the date of which again I chose the date I want. I get a sms that they debit my account yesterday and again this morning obviously with not sufficient funds available. I then call the call centre they say they can only change the date and nothing else. I must go into store to sort out. I WILL NOT BE LIABLE FOR ANY FAILED DEBIT ORDER CHARGES EVER! This happened at Vangate Mall Telkom and I have my contract with the woman who assisted me of which she should be liable for because she inserted the incorrect debit order date.