Use cross reference directories to find missing information. Typically used for looking up phone numbers, a directory search for either the anonymous number or the user's name provides the complete information, filling in the blanks and solving the mystery.
If you are receiving calls from an anonymous number, you can refer to a free online reverse telephone directory to reveal hidden information, such as the location of the call and the name of the person connected to the phone number. Instead of returning a missed call to a long distance number, you can simply cross-reference to find the person online and gather any further information with a Google search.
The US Library of Congress makes available to the public an extensive collection of current and non-current telephone, city and reverse telephone (criss/cross) directories for the United States and many foreign countries. The Cross-Reference Index consists of two sections: a Community Index, which identifies the directory in which a specific community's listing can be found; and Directory Coverage, a list of all the communities covered by a specific directory. These directories often provide additional information about individuals, such as place of employment and name of spouse, along with numerous other variables.