Known as an area code overlay, the two area codes will work together in the same South Bay region that includes much of the tech industry-heavy Silicon Valley that has just about run out of three-digit prefixes within a 408 area code, according to the California Public Utilities Commission.
As of Saturday, callers within the 408 area code must dial 1 plus the three-digit area code and seven-digit number to make local calls, although their phone numbers will not change and they will not be charged extra for making local calls.
The CPUC created the overlay to insure that new numbers could be added in Santa Clara and other counties using the 408 code. New numbers in the affected region will be assigned the 669 area code starting Nov. 20.
Phone customers will have to program devices such as life-safety systems, fax machines, Internet dial-up numbers, alarms and security codes to recognize the 669 area code, the commission reported.
Businesses will have to revise stationery, websites, advertising, bank checks and personal materials to include the 408 area code in their numbers.
The CPUC decided to add the new area code last year and started a six-month grace period, temporarily permitting seven-digit local calls, on April 21. As of Saturday, that period ends and callers will need to dial 1 plus the seven-digit number to make local calls.