WHY MUST I PAY FOR A CONTRACT WITH NO DEVICE

MTN has been billing me for a contract that was meant to be canceled in September as I returned a wrong device that was delivered to me. I have been trying to get the contract cancelled for 3 months now. Even worse, they even sent me a confirmation SMS that my line was scheduled for cancelation but it was never done.

Is that even lawful? How do you dare invoice someone for something they do no have