This is probably a scam, as if you owed this debt, you'd likely know about it and such scams are very popular. Unscrupulous people buy up consumer lists and listo of old debts and then try to intimidate people into paying money they don't owe.
You can check your credit history and see if there's an unpaid loan you owe money on by going to www.annualcreditreports.com. You can get a free copy of your credit history from all three credit reporting agencies and see it on the web, compliments of the Federal Trade Commission. The site is government approved.
You also have substantial rights under the Fair Debt Collection Practices Act. In the first place, you can demand that they validate this debt in writing so that you can dispute it. Also, under both state and Federal law, it's illegal for a debt collection company to harass you or threaten you with prosecution for failing to pay a civil debt. So what they are doing is harassment and intimidation and you can and should report them for it.
You can dispute this debt, which you should do in writing, by sending the collection agency a cease and desist letter, letting them know that they are in violation of both state law and the Federal government's Fair Debt Collection Practices Act. Tell them to immediately refrain from contacting you or anyone else such as your family or employer or that you will report them on the state level to your state Attorney General and on the Federal level to the Federal Trade Commission. Remind them that from here, every violation could get them fined $1,000.
If you really owe this money, they will have to serve you and take you to civil court. But if you don't, this should be the end of it all. The Federal Trade Commission gives a wonderful overview of overaggressive and/or fake collection agencies and how to get them off of your back, where to report them, where to learn more, etc. And it's pretty straightforward. You can access that here.