MTN RETURNED PHONE

I bought/took a contract phone from MTN on 28 January 2019, which I returned on 31 January 2019 because I was unhappy with the package of 5 different sim cards they gave me. The phone has not yet been taken out of the system, and I am struggling as my credit record is affected. I am harassed by debt collectors daily with the same account, which tripled from the original amount. I have proof of goods return, but MTN has not yet taken me out of their system, and this is unfair as I have returned the phone. I dealt with three managers at the shop for over 4 years, and they cannot resolve this issue. MY ID number is blocked and on the credit bureau, and I cannot buy anything due to such a simple issue that can be sorted out within minutes. I went to the police station to request an affidavit to prove that I did not owe MTN, but they kept changing Debt collectors companies to call me and did not want to accept that I did not owe them.