Cancellation of a contract

Been in a contract with Mtn for about 2yrs, it expired last month. To my surprise I got data again even after it expired and when I called, I was advised I had to cancel it if I don’t want the contract anymore. And the bigger shock was that I had to pay more than I’ve been paying cause the contact is expired! Okay. I’ve been trying to cancel it for over a month, their cancellations department phone is never answered. I want to cancel cause the money they’re now debiting is too much and it’s not worth it, why do they make it so hard to cancel an already expired contract? Why can’t they just cancel it if you don’t renew it?