New Debt and Cell C harassment

I'm having a torrid time with Cell C and New Debt Solutions. Early last year, my contract was handed over due to non-payment. I was handed over to two different EDC's (SSDA and New debt solutions.) 6 months later I paid back the money I was behind with and continued paying my normal instalment. Because I was 1st handed over to SSDA, that is where my payments have been going towards. Late last year, I got a call from New Debt stating that my account is behind. I advised them that I have been making monthly payments and that they should double check their records. I sent them my POP's to verify the payments I've made. Every single day New Debt is harassing me whereas they have the incorrect balance of which I told them to rectify, they still haven't rectified. Their agents are very rude and have no respect for a client. My account is up to date and my last payment is due in July yet, I'm getting emails stating that I will be listed adversely.