Cell C billing dispute

I returned a Samsung A10, sent to me as contact upgrade, to Cell C in September. I contacted then immediately telling them that I did not want the device. It took Cell C two weeks to process the collection despite numerous calls and emails. Once they collected it, I contacted them to find out if the credit notes have been issued; on both occasions they said yes. I have not received it. My October invoice still reflects the charges for: *** for R142.***05 for R306.39 The contracts advice were approved and sent to me by Cell C without my payslip, proof of address and bank account authentication. How is this possible? The service has left me unhappy causing me to cancel my 14 year old cellular contract.