Cellphone contract ended, yet profile handed over and I am still being billed while my credit score is being damaged

My contract with them ended on 13 September 2023. I submitted my notice to terminate the services. I was incorrectly billed by double my regular installment. and that sent my account into a deficit. I reversed the money, they handed me over to Nudebt. I paid the "money owed" to telkom. I submitted the proof to Nudebt. Yet I keep receiving Notice of demands, section 129 etc. Also my telkom account says I owe yet another monthly installment when my contract has ended with them. STOP messing up my credit profile! I regret ever taking a contract with Telkom. There is no easy way to contact this company, and when I finally get through the staff tells me different stories regarding the status of my account.