Blu label connect ***********

I took a contract in 2020 and they sent me a phone that was not working properly so I called them the next day to cancel the contract and take the phone back. The phone was collected 2 days later and I was told the contract was canceled. After that they kept on calling me and saying I didn't pay my installments I explained to each agent that I have canceled the contract, they checked up on it and said it hasn't been canceled then I was put through to management and the lady spoke with me I explained the story and she said she will cancel it. And it has not been done till date l. Today I received an sms from a debt collection company stating I owe R7656. This is ridiculous how can I pay for something that I have returned and asked for it to be canceled numerous times.this will affect my credit score all because people can't do their job properly