MTN is a **** and rips people off

My contract with MTN ended and I wrote a cancelation letter as per their request however they have been billing me since and when I called to enquire they stated that it is for MTN insurance. I called the insurance that confirmed that my insurance with them was canceled over a year ago yet MTN still bills me. The insurance also confirmed that the billing is from MTN however MTN insists that my contract with them is canceled. I am not getting assistance and I paid an amount of R3700 just to clear Arrears that they listed me for while I have no contract with them