When the
American Telephone and Telegraph Company (AT&T) devised the first nationwide
telephone numbering plan for
Operator Toll Dialing in 1947, California was divided into three numbering plan areas, which were reorganized geographically in 1950. The first area code split became necessary in 1951, when most of the southern and eastern portion, including San Diego and most of Orange County, was assigned area code 714. In 1982, 714 was split and almost all of the southernmost portion of California, from
San Diego to the
Nevada border received area code 619, the first new area code in California since 1959, when
707 was added. On March 23, 1997, most of outer northern San Diego County, desert areas and geographical areas in southeastern California were removed from the numbering plan area by a split for
area code 760. On June 30, 1998, the NANPA approved a request by the California Public Utilities Commission for a two-phase three-way split of 619 such that the first phase would introduce the new 858 area code to
northwest San Diego County on June 12, 1999, after which the
southern and
eastern parts of the county would be split off into a new 935 area code on June 10, 2000, leaving the remaining portion of 619 to serve almost exclusively the city of San Diego. However, after the 858 phase of the split, popular and political opposition throughout the country regarding the large number of area code splits being scheduled forced the NANPA to implement 1000-block number pooling, which effectively extended the life of the remaining portion of 619, and the 935 phase of the split was cancelled. In response to projections that 619 would exhaust its central office prefixes in March 2019, the
California Public Utilities Commission approved a relief plan that eliminated the 619/858 boundary, turning 619 and 858 into overlay area codes for all of the inner portion of San Diego County. This change took effect in June 2018; at that time, ten-digit local calling became mandatory. At the time, 858 was not projected to exhaust for at least thirty years, despite North County's continued growth. Under the most recent projections, San Diego will not need relief until 2043. ==Service area==