disgusting service by markhams

I paid all my installment for my phone and insurance in full until i was owing nothing and then I cancelled my insurance as I had sold the phone. I went to markham in fourways and was given a cancellation form to fill up and was told they will fax it and it takes only 24hrs to cancel. Now 2months later markham sends an email that i'm owing and I mus pay before they take legal action against me,i find it unfair to me to pay the amount as i had done my part in time. Their customer service line is unhelpful whenever I call. I strongly believe markham staff never faxed my forms and now they make it my fault,pliz help