ITCBA payments not being updated ad I am harrassed for not paying when I am paying and send proof.

Terriblle service! I have been repaying a Discovery Health Account through ITCBA. For years I make a monthly payment to the same public beneficiary account via my fnb online banking. No problems - I even received monthly calls to thank me for payment. Now, the last six months or so, I have been accused of not paying. I send proof of payment yet I get harrassing calls. Apparently payments are not being updated to my account. This is despicable. I have requested call centre agents to escalate the matter and contact a manager - to send me written correspondence about the issue via email - to no avail. I just get a new agent calling me every month telling me I have not paid when I have. Ref = ***