No invoices + no statements = no payment. But account gets handed over.

I am employed by a corporate client of Afrox. On this account, we are billed for cylinder rentals - supposed to be invoiced and paid monthly. We have not received any invoices or statements since March 2019. They have since handed our account over to MBD for collection. I have been taking these calls (weekly) from collection agents, and in every conversation I have, I have asked for the invoices and statements to be sent to me. After concluding each of these calls, I email MBD and Afrox (again) asking for the same documents. I never receive any reply or documents, but they diligently keep calling. How do they think this matter will be resolved?!