No Correspondence - Account Handed Over

We had an unpaid account from Ampath that was handed over to Lexmed. First of all, Lexmed were confused about which account it was that was unpaid. Secondly, I queried this outstanding account as we had no correspondence from anyone regarding this account before it was handed over. I have asked on several occasions for proof of delivery of statements and no one has bothered to come back to me. It is not right that this account was handed over as we were not aware that it was unpaid.