Contract cancelation

I had a 3 years contract with them after paying for that 3 years they continued to take my money I called them they told me contact has changed to month to month contract then I asked them to cancel it since I don't need their services anymore after 4 months of cancelation they called me telling me I owe them an amount of R1600 I asked them how they told me my contract was never canceled so I'm asking myself if this is how they operate Now my credit score is affected because of them