Poor service

Years back I took a contract phone with 8ta,the contract was for 2 years.I paid them all their money within that period.So now I have been getting emails from debt collectors saying I owe Telkom +-R2000.I tried calling them on several times,they keep on saying the will investigate the matter and get back to me.Until to date they have not.I want TELKOM to clear my name as it is affecting me my credit score.