I regret taking a contact at Cell C

I regret taking a contact at cell c I had a contract with Cell c and one Saturday morning I went to the branch to cancel the contract and paid a settlement amount of R1700 and that was February/March 2020 then months later i got a call from cell c consultant saying i haven't been paying my account. I have been trying to call them explaining my situation and I even went to the branch and even today no one listens to me. Apparently the R1700 i paid was used to pay for my installment upfront. I even have proof of payment that I paid at pep store. I don't know what must I do because the consultant didn't send the POD to HO for settlement. I'm not at fault it's because of their bad service.