Area codes 510 and 341

Area codes 510 and 341 are telephone area codes in the North American Numbering Plan (NANP) serving much of the East Bay in the U.S. state of California. They cover parts of Contra Costa County and western Alameda County, including the city of Oakland, but excluding Dublin, Livermore, Pleasanton, and Sunol.

Area code 510 was established on September 2, 1991, in a split from area code 415. On March 14, 1998, the inland portion of the East Bay was split off as area code 925. The dividing line followed the Berkeley Hills; almost everything west of the hills stayed in 510, while everything east of the hills transferred to 925.

In response to projections that 510 would exhaust in the second quarter of 2019, the California Public Utilities Commission approved the addition of an overlay area code, 341, to serve the East Bay. The first central office codes in 341 became available on July 22, 2019. Ten-digit dialing became mandatory in the East Bay a month earlier, on June 22.

Service area
The service area contains the following cities in two counties.

Alameda County

 * Alameda
 * Albany
 * Ashland
 * Berkeley
 * Castro Valley
 * Cherryland
 * Emeryville
 * Fairview
 * Fremont
 * Hayward
 * Newark
 * Oakland
 * Piedmont
 * San Leandro
 * San Lorenzo
 * Union City

Contra Costa County

 * Bayview-Montalvin
 * Canyon
 * Crockett
 * East Richmond Heights
 * El Cerrito
 * El Sobrante
 * Hercules
 * Kensington
 * North Richmond
 * Pinole
 * Port Costa
 * Richmond
 * Rodeo
 * Rollingwood
 * San Pablo
 * Tara Hills

Introduction of 341
By 2016, the California Public Utilities Commission (CUPC) projected that NPA 510 would exhaust its numbering pool by the second quarter of 2019. In response, the commission held meetings in Berkeley, Oakland and Hayward in January and February 2017 to discuss area code relief for 510. The North American Numbering Plan Administrator (NANPA) proposed in May 2017 that CPUC relieve 510 by introducing the overlay area code 341 for the entire territory. NANPA had previously assigned 341 as the relief area code for 510 in 1999, but nationwide number pooling procedures eliminated the need at that time. The NANPA retained 341 for the then-future relief of 510. CPUC accepted the NANPA proposal in a decision on June 21, 2018. With the start of the overlay plan in July 2019, all customers in the numbering plan area can be assigned telephone numbers with either code, and must dial the area code for all calls; otherwise, a recorded message will remind them if they dial incorrectly.

Prior usage of 510 for TWX
In 1962, AT&T assigned NPA 510 for conversion to dial service of the Teletypewriter Exchange Service (TWX) in the United States. Telex use of area code 510 was decommissioned in 1981 after conversion of the service to new transmission technologies by Western Union,