Isn't there only two states where "as long as the PDl is registered somewhere then they can go by the laws in that state."? But you aren't in one of them, so yes the laws in your state are what matters. You keep telling them that, be firm, be strong, don't let them intimidate you!
And check your credit report. When you get notice from a collection agency send the dispute letter, there is some wording when dealing with them that you have to be careful of so search and read up about dealing with CAs.... If they can't validate the debt and you find it on your credit report then have those copies of your validation letter along with the return receipt from when you sent them to the CA, and you'll probably have to use those to help get it off your credit report (proving they haven't validated the debt and thus must remove it).