Contract cancellation

I've have a phone contract with cell of 24 months my contract was supposed to end in May. In June I received an invoice from them and they deducted money, I contacted them and asked about they said I should've requested for cancellation of contract and I did in June they said it will take 30 days. In in July I paid my 30 days notice and to my surprise August I received another invoice of 350 this time November I called again to cancel Iwas told about the 30 days and my last deduction will be December and today in January I have just received a statement stating that I need to pay R250 end of Jan now I don't understand how because I've already requested them to cancel my contract in November and gave them 30days notice which was in December how is it that now i must pay in Jan again? I just called again and the customer agent is telling me they can't find any cancelations on her system. Cell c is a ****** ****.