My mtn account

I have a contract phone with mtn for many years now in last year my account went in arrears and my account was handed over to MBD in February 2017 I went in to mtn to find out what the amount due was I was told my account is no longer with mtn and they put me on the phone with MBD I was given an amount if +- R4400 I can't remember the correct amount then I went to abs to pay the amount I was given reference number was my cell number then after mtn open my phone and I was told that my account is back with mtn and I have been paying my account every month since then.... two or three months later I have been getting no stop calls from MBD say that I owe the +-R5000 I have spoken to mtn customer service and I was told that my account is up to date and I need to tel that to MBD wen they phone again in fact I phone MBD back bt still they didn't believe me mtn has emailed my all my payments that I also send to MBD and yet I am still being phone I have explained this story so many times and I am really sick and tired about this service I don't know why MBD can't phone mtn regarding my account with them and I don't know why mtn can't phn MBD to set the record straight telling them that my account is up to date