According to an FTC website:
Fraud alerts can help prevent an identity thief from opening any more accounts in your name. Contact the toll-free fraud number of any of the three consumer reporting companies to place a fraud alert on your credit report. You only need to contact one of the three companies to place an alert. The company you call is required to contact the other two, which will place an alert on their versions of your report, too. If you do not receive a confirmation from a company, you should contact that company directly to place a fraud alert.
TransUnion: 1-800-680-7289; www.transunion.com; Fraud Victim Assistance Division, P.O. Box 6790, Fullerton, CA 92834-6790
Equifax: 1-800-525-6285; www.equifax.com; P.O. Box 740241, Atlanta, GA 30374-0241