Yes. Write back. Their confusion has nothing to do with whether you owe the bill, or not. Focus on that. Write again saying you do not owe the bill (wherever you live :)
This is general legal information, not intended to apply to your specific case. And I may not be licensed to practice in your particular state. Under Federal Law, I am a debt relief agent.
I would provide your proper address - feel free to contact my office next week and we can try to put you in touch with a local attorney who can help you. My best to you and your family.
Please do not take my answer to be legal advice that would establish any attorney-client relationship. Please take it as a general response from my own experience in response to your question. I hope you find it helpful.