Cannot keep to an arrangement and keep on stalking me and wasting my time every month

I am at awe with the service at Teljoy. I had issues in the past few months with the incompetence of the staff of Teljoy. I then made an arrangement to pay of in instalments the extra charges for rent which was charged in advance but no one thought of mentioning this to me. Last month Teljoy went and deducted the total arrears from my account eventhough I made an arrangement. I spoke to the accounts department who again loaded this arrangement on the system. They also promised me that it will not be a problem again as I have to waste my time every month to sort this out. AGAIN I now received threats that I should pay the amount immediately and today is the 25th of the month and I received a threat regarding my outstanding account which is only payable the end of the month. I CANNOT believe that I AGAIN have to waste my time because of your incompetence. Is it so difficult to load an arrangement on your system and keep to it? If you cannot even keep to your own arrangements how am I supposed to honour it? I am really at the point where I just want to cancel this lease as it is just causing me difficulty every month. It is ridiculous.