Debit Order due date - Not changed after repeated requests

I took out a contract on 2 cell phones and when they were due to be upgraded, I cancelled my cell phone contract but took a contract out on a lap top. At that time I asked the sales person to change my debit order due date from the 25th to the last day of the month. I physically saw him make the change. If I remember correctly, this was January/February. To date my account is still debited on the 25th of the month. I have made numerous calls and still no joy, It's now been more than 6m and still not resolved. Here are a few reference numbers received from CellC regarding my issue, *** (April) *** (May) *** (May) *** (June) No one has even bothered to call or email me. Please can this get resolved now please, it is ridiculous that for more than 6m a debit order date cannot be changed. It is unacceptable and shocking to say the least!!