cancellation of contract

I called and cancelled my contract with mtn early this year they gave me a ref no:*** and the debited my account end of the month. The next month I found that am owe the m R 514 I went to 1 of their branch to complain and to find clarity about that money course I knew that I pay R199 p/m and my account was cancelled and they told me they can't help me because they cannot call the call centre because of the strike. I went to other branch sunny-park they help me and closed my contract and change my contract to prepaid Ref no:*** and since then I never received anything from mtn, so today I was surprise to receive a call from their law saying im owing them. I went to the branch and they check on the system and try to call 808 and its show that I don't have active contract with mtn.