Getting Billed on something I don’t use

Good Day, I had a contract with MTN for two years and when it’s time to cancel they tell me I have to pay the remaining amount. I would have paid but now I’m getting billed 1500 every month. I went from paying R400 to R1500. Apparently I’m on month to month. I keep telling them I want to pay the remaining money but they keep billing me. It’s really unfair expecting me to pay something I don’t use. MTN cancelling process is bad