Apple iPhone blacklisted after cancelled contract

Hoping that you can please assist us with this problem that we are struggling to get a solution from you. My contract with Cell C ended and when I tried to trade my iPhone in at the Apple store, they informed me that it is black listed. At no point did I ask for this phone to be blacklisted. Why was it blacklisted given that I own the phone as it is fully paid for? I was on pay as you go as the 2 year contract had ended and my number was ported over to Telkom. The networks @ Cell C are the reason we cancelled as the signal is very bad.

Several unsuccessful calls to Cell C, calls being dropped and generally unhelpful service. Finally today they tell me that they cannot find my phone number or the IMEI of the iPhone. This is impossible. Surely records are kept. I still have 2 phone numbers on the same account with Cell C.

I need this phone to be removed from your black list by the end of this weekend please.