Beginning this Saturday, June 22, all calls made from 510 area code numbers will only be completed if the caller dials “1”, plus the 510 area code, then the seven-digit number. This is a result of the California Public Utilities Commission (CPUC) adding area code 341 to the 510 area code region that encompasses the majority of Alameda County and the western portion of Contra Costa County. 

Calls that are dialed without the “1” and the area code will not be completed and will result in a recording instructing the caller to hang up and dial again, according to the CPUC. This includes both cell phones and land lines. Current 510 numbers will not be changed. The 341 area codes will be assigned to new phone lines beginning July 22.  

The CPUC is adding the 341 area code because the 510 prefix (the first three digits of a seven-digit number) numbers are projected to run out sometime this year. 

The North American Numbering Plan Administrator (NANPA), the neutral third-party area code relief planner for California, filed a motion with the CPUC to add the new area code on May 19, 2017, according to a memorandum from CPUC. CPUC held public meetings on Jan. 17 and Feb. 15, 2017, in Oakland, Berkeley and Hayward. At the meetings, NANPA presented the overlay and split options for adding a new area code to the 510 region. A majority of the public comments indicated support for the overlay option.

In the split scenario, the 510 area code region would be divided into two regions, one keeping the same area code and the other changing to a new area code. This would result in a significant amount of current 510 area code phone users needing to change their phone numbers. In the overlay scenario, a new area code is added to the current area code region and phone users can keep their current phone numbers. However, for multiple area codes to exist in the same region, phone users must dial “1” and the preferred area code before making a call.

The overlay plan was approved on June 21, 2018. 

Seventy-six percent of all 510 area code phone numbers reside in Alameda County, according to 
allareacodes.com. The other 24 percent reside in Contra Costa County. 

The 510 area code was created when it split off from the 415 area code in Sept. 1991. The 510 area code was split in 1998 to create the 925 area code, which serves eastern Alameda and Contra Costa counties.