Billing for the wrong item

Good day, one of the Cell C agents called me offering a contract phone. I accepted the contract, but a wrong phone was delivered. So they said I have three days to return the phone if I'm not satisfied, I phoned them the same day saying the phone was wrong. The person I spoke to claimed to log a call, I can confidently say I called them more than ten times, but was sitting with an un-opened phone for three months, waiting for them to come collect it. They have been billing me and I was reversing the money till they stopped. Now they sending me massages that my Cell C account is overdue and I must contact NDS to arrange payment so I can prevent further action being taken. How can I pay for a wrong item of which I never even opened or used. PLEASE HELP!