MTN funds reallocation request is being ignored

I have 2 accounts with MTN: Account X is paid via debit order whereas Account Y is paid manually (not sure why). MTN have erroneously allocated funds for Account Y to Account X. I have asked numerous times for the invoice to be credited and reallocated correctly to no avail. Instead of a resolution I get emails saying Account Y is in arrears. I am getting more and more frustrated that a simple request seems impossible to accomplish and in the meantime my credit rating is being negatively affected.