No help blacklisting phone

Have been sent from pillar to post trying to blacklist a phone. Not to mention at least 2 hours holding on and speaking to people who are passing the buck from dept to dept. No one seems to know what to do.

I took out a contract with cell c (received a cell c sim and new cell phone), the contract ran its full duration and eventually I opted to go prepaid in Feb 2018. This month I have lost my cellphone and need to blacklist it in order to claim from my insurance for a new one . Now Cell C claim they cant blacklist the phone as I am no longer a customer. They do however still have my details and know which device I took and when. Surely because I took they device with them they should be able to blacklist it? My new network provider cant blacklist the phone either because I didnt take the phone with them. Only a prepaid sim (so they dont even know what phone I have been using). Now I am in a position where I cant blacklist the phone, and thus cant get a new one because Cell C cant block it. Please help