Because the vast majority of identity thieves open accounts using your stolen information, those unpaid balances wind up being placed with third-party debt collectors. Armed only with your social security number, collectors are able to use online databases to obtain your contact information, including phone number and mailing address. If you get a phone call or letter demanding payment for an account you don't recognize, here is a list of steps you should take:
Whether you get a phone call, receive a letter, or notice an unfamiliar account on your credit report, the very first step you should take is to mail a written dispute to the collector. The letter doesn't have to include any magic words. Just tell the collector you believe the account might be fraudulent, request any and all information in their possession regarding the account (known as "validation") and ask that they only contact you in writing. Send the letter via certified mail to insure its receipt.
When a collector contacts you, they'll give you what's known as the "mini-miranda"; i.e. the communication is an attempt to collect a debt and any information will be used for that purpose. Take that admonition seriously. Smart collectors will listen to your claims of fraud and ID theft, but there are a lot of...not smart collectors. If they start asking you rapid fire questions, resist the natural urge to answer. Even better, end the call.
Not everyone is comfortable turning the tables on a debt collector, but if you are, do it! Find out the name of their client, when the account was charged-off, the balance of the account, the original mailing address on the account, whether they have billing statements, an application, documents with a signature, etc. etc. There are collectors who can't help but try to impress you with the amount of information they have, so let them show off. Interrogate them rather than be interrogated.
Believe it or not, there are decent collectors who would prefer not to collect money from people who don't actually owe the debt. In those cases, the collector will ask you to fill out a fraud affidavit in which you swear you're a victim of identity theft or request you sent them a police report in which you've reported that your personal information has been stolen. In most cases, this is a bona fide attempt to get to the bottom of what's happened and is the most efficient way to get them to leave you alone.
Keep all mail correspondence collectors send you, make copies of all letters you send them, and log every single phone call. For a call log, note the date, time, company, name of representative, number from which they're calling, and the substance of the conversation. This goes doubly if you've told them not to call you and they do so anyway. Also, try to determine whether they're calling you using automated dialing equipment (robo-dialer).