Frustrated customer

My account was handed over by mtn to ssd . They kept on increasing the amounts owed. I later on requested a settlement amount of which I paid instantly but now I'm being ignored for the settlement letter that I want to provide mtn with to close of the account and not continue to bill me .the next best thing the will be hidden fees that I need to pay since they dont want to give me this settlement letter. I made numerous calls with no avail and emails that only gets refunded now and never returned.