A new system for dialing calls in the 510 area code takes effect on Saturday.
To complete calls to and from numbers in that area code, dialers must begin by punching in 1 plus 510 before entering the remaining seven digits, the California Public Utilities Commission said.
This change is to prepare for the introduction of a new area code for parts of Alameda and Contra Costa counties.
On July 22, the 341 area code will mingle with the 510.
No one will have to change numbers when the 341 debuts, but it will be assigned to some people opening new lines. Some new customers may receive a 510 area code.
Numbers in the 510 are close to exhaustion with nearly 8 million phone lines already assigned to the code.
The 510 was created in 1991 when it was split from the 415. It was split again in 1998 when officials created the 925 area code.
