Great confusion over payment!!

I've been a client for Telkom for many years. The last couple of years I've made my monthly payment via eft on the 1st of each and every month without any problems. It's now about the 4th time I randomly receive a bill almost twice the actuall amount due. When contacting them they tell me that they've tried to deduct the amount via debit order and that's after I've already made my payment and chose the pay and clear option in order for them to receive their payment immediately. I've spoken to 3ladies before and their reply was that I need to pay the full amount because they weren't successfull to retrieve funds from my banking account. The other reason they gave me is that they receive they payment too late on their side when I make a payment on the 1st as I now all of a sudden need to make a payment 10working days before month end should I like to pay via eft. I did inform them previously that Im only making use of eft's,there is no debit order and that the banking account they are using has been closed more than 5years ago. Now my question is why were there no problems before for years doing it this way? Who instructed them to all of a sudden place a debit order sometimes when feeling like it after it hasn't been done this way for many years? How is it even possible to receive a late payment as I always pay & clear on the 1st? I haven't even been with the bank for many years whose details they've got and try to deduct it from. The lady I spoke to today said I need to go into a store and ask them to remove the debit order from their system. Why should I after having no payment issues for years? She also said I should pay the full amount. Why must I as the mistake is not from my side and I can't be held accountable for the banking fees of an account that does not exist and a company who hasn't been instructed to all of a sudden when feeling like it decides to place a debit order?