Cell C wrongfully Blacklisted me

I had a contract with Cell C around 2016 which I closed when I moved to another telecom, boom this year when I check my credit score I found out that I owe Cell C R30, I contacted them and they refer me to the collection department, I paid the R30 and then they called me saying they don't see any record of me owing them money. Now I am stuck with R30 that appears on my credit record which they can't help me with. Can anyone help me with this R30 that I owe, to whom I do not know?