Contract Being Cancelled without alerting costumer

I had contract with cell 2 years back the close my line because I had arias due to bank account changed, but was my arrangement has been done and I finished pay arias and i was also paying premium, they cancelled my contract claiming I can't afford the contract anymore, my airtime and all things that i was paying for was forfeit and i made peace with it move on as they gave my number to someone, a month after they came with invoice saying cancellation fee of R3500, and i ask them why should i pay cancellation fee if i didn't request cancellation and how can someone not afford to pay contract while i getting airtime and data but been cancelled and pay it without getting any thing, when i call again one told me is ford device and device that i took was costing around R2600 new, i had it for 9 months. even now i didn't get the real answer, some one just want to score points and push me in the debts now, if i go to cell c nothing in the record the case is sitting with debts collector which they always negotiate payment only, and my credit score is mess up because of the case. I need them to clear that on my name, or talk to business.