Contract ended but handed over.

I have been getting calls from SSDA and being harassed for an account that was handed over by Cell C. I have never missed a payment with Cell C and now I am getting calls about a closed account being handed over for +/- R7000.00

My contract ended in November 2020 and all monies were paid to Cell C. Mid June 2021 I get a call from SSDA. I have called the consultants at Cell C and they have told me they cannot find anything on the system. I asked SSDA for a statement for this R +/- 7000, instead they send me a sms with the amount, no information, they just want money.

I know for a fact I do not owe Cell C R 7000. I want some answers from Cell C. Why was an account which had ended handed over?