MAINWAY is a database maintained by the United States’ National Security Agency (NSA) containing metadata for hundreds of billions of telephone calls made through the four largest telephone carriers in the United States: AT&T, SBC, BellSouth (all three now called AT&T), and Verizon.
The existence of this database and the NSA program that compiled it was unknown to the general public until USA Today broke the story on May 10, 2006. It is estimated that the database contains over 1.9 trillion call-detail records. According to Bloomberg News, the effort began approximately seven months before the September 11, 2001 attacks. As of June 2013, the database stores metadata for at least five years.
The records include detailed call information (caller, receiver, date/time of call, length of call, etc.) for use in traffic analysis and social network analysis, but do not include audio information or transcripts of the content of the phone calls.