In 1997, the California/Nevada Code Administrator (C/NCA) advised the North American Numbering Plan Administration (NANPA) of the need for area code relief in the 408 numbering plan area.
Area code 408 would be retained in Sunnyvale, San Jose, Los Gatos, and Gilroy.
The area code split became effictive on July 11, 1998, with a permissive dialing period ending on February 20, 1999.
[4] The service area of NPA 831 includes Big Sur, Monterey Bay, the Salinas Valley, and the southwestern Santa Cruz Mountains.
Major cities in the area are Salinas, Hollister, Monterey, Santa Cruz and places in the northern Central Coast.