Debit order deducted before contract date

I just received a sms today from telkom mobile saying that my debit order which usually goes off on the 25th of each month will be deducted from my account in the 24th. I phoned the call centre 081 180 on 20 April 2015 at 17:44 and spoke to a Susan to tell them they cannot do that as there is a reason why one chooses the 25th or which ever date because you know that is when you receive your monthly income. She then told me if it falls on a weekend they either deduct it the first working day before or after the selected date. I then told her but then it should be the first day after because you cant deduct money from someone before the date they selected on their contract. I requested to speak to the billing department which she told me I can't. Then I requested to speak to her manager and she told me he is busy on another call and he is too busy to speak to me afterwards. Why do they give you the option to phone if you have a query about it but won't change it?? If this is the case why do we even get the option to select a debit order date when they do what ever they want anyways? This is unexeptable and I am not letting this just go! If my debit order bounces they will refund me