Can I please be given a paid up letter because I'm not owing you

Please assist

I had a contract with mtn for over 8 years now,I failed to make payments and was handed over,was called by the lawyers given an amount I owed made arrangements for payment and paid, I then emailed proof as agreed nd then waited for my paid up letter.When I ddnt get the paid up letter,I then called the company was told Mtn says I still owe them so I must call mtn. I then called MTN who gave me a certain amount nd account to pay to.I made the payment sent proof and asked for paid up letter.Called MTN was taken from pillar to post until I finally gave up because I never reach the legal department where I supposedly being trànsfered to.

Its been over 6 months now and I am not being assisted,what other steps can I then take?bcoz its making my credit score look bad nd already they have given my number to someone.