Bad Communication

I went into cell c on the 18th of April to give them my new banking details for my contract debit order. I did the same thing with my game account. When I got paid, my game account money was deducted but I saw that cell c hadn't yet come off. I purposely have not spent the money knowing that the debit order is suppose to come off. Today I receive an sms telling me that my debit order has bounced and that there is also an outstanding fee of R70 because of this. I then phone the cell c branch that I went to and spoke to someone I know that works there. He tells me that the lady that helped me should have told me that it takes 30days for the new account to be active. If I had known this, I would have then kept my previous bank account open and transferred the money that I know I pay every month into that account. Now it is up to me to pay them and to still pay the extra fee. Why is it now my fault? I made a point of coming in a week earlier to give them time (not knowing it actually takes 30days) and now I'm being charged as if this is my problem. I don't see why I should be paying For someone else's mistake.