Cell c wants me to pay for services not rendered

Back in 2018 I think i took out a contract to get fibre from cell c via the metro fibre line thats installed in midrand. The line was never moved from metro fibre to cell c fibre hence I continued to use metro fibre. Cell c now wants me to pay R8000 for a service they never provided, of which I got from another vendor and paid for. They have now listed me as defaulting on the account of which they have never tried to recover with me. Once contacted I told them of the issue and they said they would escalate it. Until this day I have not gotten a call from them. Mind you I have two other contracts with them that I pay for every month. After years cell c wants to ruin my credit profile, I regret ever using them.