Collection lawyer threatening to sue

Hi. I hope you guy can help me. My debt is 30K and is now being handled by the lawyer for Riverwalk Holdings. The lawyer is threatening to sue, and in fact they claim they have already served me, which they have not. I have never been in this situation before, and I have no idea what to do. I'd like to offer a lump sum payment, but I don't know what to begin with, and I haven't even got close to the amount of the debt. How should I go about dealing with this? I really appreciate your help.