The next step is they are going to come to your home and take your furniture, TV and any cars you own and then the cops are going to take you to jail lol.
I believe your questions have been answered and just ignore their calls and they will stop in due time. The only reason they are threatening you is because they know the law is not on their side any longer. The debt will only be at their agency for six months and it will get sent back to the creditor.