The San Jose Mercury News says new phone lines from Sunnyvale to Gilroy will get a 669 area code.
The state Public Utilities Commission approved the plan Thursday.
The area code overlay begins in November 2012.
Everyone in the 408 area code will keep their current numbers.
But a year from now, everyone in the 408 area code will have to dial 1 plus the area code before calling anyone, including other 408 numbers.
The 408 area code is running out of numbers.
The 669 area code is expected to last 36 years before it runs out of numbers.