Acount Paid for the next 4 months but still billed

Beginning of October I paid R800 into my contract account. The plan was that they will debit the monthly R189 from this money for the next 4 months. 25thOctober the money was taken from the R800. Which left R600 on my Cell C account. However in November this did not happen, I was billed from my personal account. I logged case with them on the 26 of November to inquire about why I was billed and to find out where my R600 went but I could not get the right answer and a case was logged reference number: ***. 7 days later I still did not have a response from Cell C. I called again to log another case reference number *** and I spoke to a guy named Senzo who told me there was an \adjustment"on my account but could not explain what that is to me. How is an adjustment done on my account without my consent. He advised that the case will be attended to in 48 hours. I wrote to HelloPeter on the 3rd of December 2015 and was told I will be called ASAP. No phone call was made to sort the issue out. I called again and spoke to a lady who said she will do a follow up. Today"