Cell c is ripping us off

I had a wifi router contract with cell c, which it came to an end in March, the following month I was billed and i called in to ask why they took money from my account even when the contract ended, they told me I need to call in and let them know that I want to cancel, which I did and was told that they gonna still bill me for the next month as i made the call at the end of the month. They following month it was indeed cancel as i stopped receiving the wifi data bundles and they stopped billing me. Now 5 months later I get a call today that I owe cell c almost R500. I think we are being **********