The 707 area code is located in northwestern California and covers Santa Rosa, Vallejo, Fairfield, Vacaville, and Napa. It serves portions of Del Norte, Glenn, Humboldt, Lake, Marin, Mendocino, Napa, Solano, Sonoma, Tehama, and Trinity Counties. The 707 area code was created in 1959 when it split off the 415 area code. It is the only area code that serves the northwestern part of California. The 707 area code is expected to be depleted of available prefixes by December 2023, and a new area code, 369, will be added as an overlay to provide additional numbering resources for the 707 area code geographic region.