NOT UPDATING RECORDS

I paid my MTN account up in Full, I received a settlement balance on my account of R6402.09 which I had paid up in 1 debit order and I have my paid up letter. Today I get told I am handed over by MTN as my account is R600 in arrears. how on earth is my account in arrears when I paid the settlement amount given to me????? I refuse to be handed over and pay that balance because I made the effort of calling and asking for my settlement balance to close my account and I have done so with the amount given to me. Nobody has communicated to me that there is another balance of R600, so this is not my fault but MTN's negligence. When I call MTN, then the call gets ended, the line is dead, machine says the number is not available, call centre is no help because they always transferring the call to a new department. I found all this out when applying for a new contract with MTN which i am waiting for feedback on how long yet all the issues is MTN's fault. guess I will not be a new customer this round!!!!!!