Unsatisfied Customer

I am not happy at all with the service provider. I had a contract with the service provider and my contract was due for upgrade in August, as per orders I called in July ( a month before my contract expires) to inform them that I would not be upgrading and requesting that I be moved to prepaid. Their employee didn't do as per request which resulted in me getting my September benefits I called again asking them why I'm still getting my benefits ok fine the matter was sorted out and my contract was changed to prepaid on the 2nd of September. Now I get my bill saying that I owe them R320 for September. Why am I being charged for a mistake done by one of their employees whereby they have a recording of my call stating that I won't be upgrading my account? The email I have received is stating that my account was suspended on the 2nd of September whereas I called a month before my contract expired so whose fault is it mine or their employee's?