OAKLAND — The new telephone area code for a big stretch of the East Bay, including its largest city, Oakland, is 341, state regulators said Thursday.

The 341 area code will be overlaid upon — regulator speak for mixed in with — the existing 510 area code, the state Public Utilities Commission decided Thursday. This means that 510 and 341 will co-exist in western Alameda County and western Contra Costa County. The East Bay also is served by the 925 area code.

The territory affected roughly adjoins Interstate 880 from the Carquinez Bridge on the north through Richmond, Oakland, Hayward and Fremont at the south end.

The change means that even people who live across the street from each other would have to dial a 10-digit number that includes the area code and their unique seven-digit number in order to contact each other. That would be true even if both numbers had a 510 area code or both had a 341 area code.

On the other hand, the rise of smart phones means a growing number of people have become accustomed to dialing 10 digits for any phone calls made within the United States.

“This will provide additional numbering resources while minimizing customer inconvenience,” the PUC said in a prepared release.

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Why is the change happening now? It took nearly a quarter-century, but the 510 area code is starting to exhaust its numbers. The number consumption is severe enough that the PUC decided to add a new area code to the western sections of the East Bay.

The 510 area code is expected to run out of available prefixes by sometime in the April-June period of 2019, the PUC said.

“Existing 510 customers will retain their area code and specific telephone number or numbers,” the agency said.

The present 510 area code serves the Alameda County cities of Alameda, Albany, Berkeley, Emeryville, Fremont, Hayward, Newark, Oakland, Piedmont, San Leandro and Union City; and the Contra Costa cities of El Cerrito, Hercules, Pinole, Richmond and San Pablo.

People in the existing 510 territory will be obliged to dial an area code, including 341, to reach a number. Even though it’s the same geographic area, customers in the current territory will have to dial 341 or 510 to reach a phone number. People using a landline or pay phone would have to dial 1 plus the area code plus the seven-digit number.

The 510 area code came into existence in 1991, when it was split off from the 415 area code. The 510 was split again in 1998, when the new 925 area code came into existence on the east side of the East Bay hills.

“The price of a call will not change due to the overlay,” the PUC said. “Customers can still dial just three digits to reach 911, as well as 211, 311, 411, 511, 611, 711 and 811.”