Accout referred to ITC

An account was received from Mubarton via Attorneys Van Rhyns which was settled on the 31/03/2015 and confirmation of payment was emailed to the mentioned attorneys. No acknowledgement was received from them. All most 2 weeks ago I received a letter from ITC demanding payment for the same account and if not paid immediately I will be black listed. On the 26th of May I responded to ITC as well as the Attorneys who payment was made to and attached my proof of payment. No acknowledgement or response was received. I contacted ITC on the 2nd of June and spoke to Kauthar Wittle who requested that I resend the same email to her. I received a response from her informing me that she is waiting for a response from their client. I followed up again only to be informed that I should take it up with Mulbarton. This is unacceptable and need this matter resolved now. How does a Service Provider so large as Mulbarton not know what is happening in their accounts department.