Charged for the paid device

Had a 24months cell phone contract deal with Cell C. The deal was for the device, data and minutes. After 24 months, they continued to provide me with the same data and minutes which I was expecting to be charged for the data and minutes only. To my Suprise, I was charged the same instalment amount which was inclusive of the device. I disputed the amount as per my expectation to be charge only data and minutes because I was using them not the device because it was already paid up. Their response to me was, I did not cancel the contract hence they continued charging me. Cell C has handed me over to the debt collectors who threatens to blacklist me.