blacklisted cellphone

I am not even a cell c client and yet the phone i have purchased has been blacklisted. i purchased this phone on my private capacity and have been using the phone for the last 5months. its has been blacklisted on the 07/10/2019. i contacted the call centre who advised they cannot help me as i am not a cell c customer and advised i need to visit a store. i went to the store just to get the same treatment. on my private capacity i managed to check the reasons the phone has been blacklisted is because the person i bought the phone from did not pay his contract. Can someone please contact me urgently to try and resolve this matter, as i am unable to get hold of the person i purchased the phone from.