Money sharks

Money sharks - services were rendered to a dependent on my medical aid, and the bill was sent to this person. Now their debt collector and lawyer are on me about the amount owed to them. I have settled the bill, yet the lawyer still continues with their threats. Who is wrong here? Me for not paying an account whilst not having received the bill? Where did the lawyer get my number to phone me for the monies owed and why could the debt collector not phone me? Why could the service provider not phone me? Surely they had the contact details of my Medical Aid details they used to file the claim in the first place? What is this?