no response from billing division

My contract expired on 6/2/2015. Which I then changed my number to prepaid. Since end of March I have called mtn 808 number to request my last invoice to be sent following sms that I have received reflecting I have an outstanding balance. I have emailed and called but every time I am told the billing department will call me back or my query has been escalated to the billing division. This has the potential of ruining my credit report. I have now exhausted all avenues available to me as an mtn customer to try to get an invoice so I can pay to no success. The last invoice sent to me has been paid.