Wrongfully ITC listed

We had a Cell C LTE contract with cell number: *** . After numerous complaints about signal quality and data usage. (Cell C said they would send someone out to test the signal, but nobody ever came to do the tests) I ended up returning the device in it's original packaging as per Cell C's head office instructions to the Cell C retailer in Springs Mall and they said that they would cancel this account. I later found out that I was listed on ITC for the unpaid account. I requested a statement for this because I paid every month whilst the unit was in my possession. I have since tried to contact Cell C on numerous occasions in order to resolve this but they are using the "Lockdown" as an excuse not to work on my account query. So in the interim, I am faced with a decreasing credit score. I feel that they are not giving any assistance with regards to this and I am the one suffering without any satisfaction.