Changed month/month to 24-month without permission

I signed a month-to-month Data contract and they *******ly and **********ly put it through as a 24-month contract on their system. Once I realized this I began to query it. I was advised by Mohammed (branch manager at Garden Route Mall) that it was a mistake that Fifi made and assured me that I will not be held responsible for the 24-month contract. I then left believing everything was sorted out. For the next two months I was charged again, even though I had cancelled my month-to-month contract - as I was unhappy that they changed my month-to-month to a 24-month - without my knowledge, request or approval. Again I went to Mohammed and since he would not give me any further clarity, and also insisted that I am now at fault for cancelling a 24-month contract, WHICH I NEVER SIGNED FOR OR ASKED FOR OR WANTED!!! And Mohammed wants me to pay a big penalty fee. I then contacted the area manager and explained everything. He then assured me that I would NOT be held responsible for any cancellation fees, since this was a branch mistake. Since then they debited my account double and triple every month and have now handed me over to debt collectors !!! No one is getting back to me!