MBD Attorneys have no customer service ethics

We have been receiving sms and calls from MBD saying that we owe money on medical bills to National Health Lab Service (NHLS). These attorneys call every week and they seem to add interest every week cos the amount due keeps increasing because of interest. We said we paid this account but they dont care. Eventually, we confirmed with our medical aid that the account was paid and that NHLS should never have sent the account to their attorneys (MBD) in the first place. Now, we cannot get any response to our emails - as we would like a letter confirming that the account has been settled. They only have call centres in Jhb and we have to keep calling 011 from Cape Town. We are not doing that anymore and we will not pay any money until a senior manager can call us and speak to us directly and tell us exactly whats happening.