They claim to be from "pre-trial processing" and threaten to serve you with papers if you don't pay them. They had my wife's address history and even the last four digits of her social. They claim she owes for a loan from 2012 BUT she has never taken out a loan. When you call them back it goes straight to voicemail. They sound foreign and they're relentless.
Read up on your rights here, get template letters to send and also make a complaint at this government site: http://www.consumerfinance.gov/
Also file a complaint with your State Attorney General's office.
List of State AG’s offices: http://consumerfraudreporting.org/stateattorneygenerallist.php