Contact cancelled but I was billed.

I am very frustrated and irritated that MTN has decided to bill me for a contract that I cancelled within 8 hours after I received my parcel. On the 11 January I received my iPhone 8 and I was not impressed and I quickly returned the phone at north gate mall., I signed the cancellation form and after that I did number of follow up with the MTN calling and email them but I was sent to pillar to post. Today am billed something that I have cancelled WHY?