MTN continuos monthly account suspension

*** - For the past 6 months MTN cant tell me why every month on the 15th this contract gets suspended. this account is paid via EFT and the user has confirmed with MTN that the account is up to date. Both the MTN Heidelburg store and the MTN Randhart stores cant tell me what the problem is as the system shows that this contract is "Status Active". The user has tried calling 135 but noone picks up. Please can someone who knows what they doing assist.