Feeling abused by Cell C

I had a cell contract for 2 years and after that i upgrated for another 2 years and i cancelled the contract but a year later they told me i owe them but the sim card was blocked the whole time....i made an arrangement to pay and i payed in 3months and asked them again to cancel the contract but now after a year again they are busy calling me everyday saying i owe them....what exactly am i owing because their simcard was blocked