No Response for Paid up letter from MTN

MTN post paid customer service is pathetic. I have been calling MTN day in and day out to resolve an issue regarding a Paid up letter. The owed amount has been paid over 3 months ago but I did not get an response left. A. P.O.P has been sent and tried calling the collections department but not answer,If I get lucky if my call gets answered I am sent to either the billing or back to the collections department with no help. It is frustrating coz I was told that MTN legal department only deal vis email but I have been sending countless emails with no response. If anyone knows the physical address to where I can acquire the Paid up letter or anyone with this same problem please advise me.