Useless service provider

I had a contract with Cell C that came to an end in May 2021, contract was cancelled and on the 11 of August I get a call from some debt collecting compony called VVM stating that I owe Cell C money on this contract, I did not owe any money on this contract, when I ended it, no one at Cell C has any idea what's going on and not really willing to help, I will never in my live do business with this compony again, they so unprofessional, and not willing to help, I was send all over the place with no resolve.