They don't knock on doors. What they are required to do is to send you that dunning letter. It is detailed when it is supposed to have been sent and what it is supposed to contain in my other post. Failure to send that is the first violation of the law and gives you the start of a lawsuit.
These People contacted me a while back told me I owed Best Buy some Money around 400 dollars I contacted first Best Buy then my Bank to see if I owed money. It didn't make sense because I sent a computer to my friend in CA. I asked the Question why would Best Buy Ship a computer to my Friend if it wasn't paid for? Went to my bank we called Best Buy they told my bank & Me I owed nothing. So it's a Scam don't answer this scam.