RCS CTM Refusing to Close our account

Last year June (2020) we Paid up our account with CTM RCS. We then received a letter from RCS Dated 10 June 2020 stating that our account currently reflects a zero balance, as well as that the account is paid up. We then asked to have the account closed. Now at the same time we closed the Coricraft RCS account and there were absolutely no issues. But RCS did not close the CTM account and have out of no where added Interest to the account, Which was paid in full and was meant to be closed. We have tried to phone and email EVERYONE possible. To which we have had NO RESPONSE. Or if someone did answer they just tell us to email them. Every person I have managed to speak to have told me it is a MISTAKE on RCS side but no one seems to be able to resolve the issue. Because we have not paid the Interest which was accumulated on a, what is meant to be closed, account RCS have now handed us over to debt collectors. We cannot get hold of anyone to fix the issue and this is now becoming an issue on our name. I am really frustrated because they not even willing to help or answer any form of email. We cant even be put in touch with the right people.