MTN contract cancellation

I had a mobile cellphone contract with MTN which expired end of August. I called them several times in August informing them of my decision not to continue with the contract. The agents there tried to convince me to continue with contract, when I declined they forward my call to the cancellation department and they just hang up my call without helping me. Now they credited my sim with airtime and data, though I informed them of my decision not to continue with the contract. They will still expect me to pay them beyond the agreed period.