The change will affect residents of Santa Clara County and parts of Alameda and Santa Cruz counties in the 408 area code, and is occurring because a new 669 area code is being added to the region.
CPUC spokesman Christopher Chow said that if the South Bay did not add another area code, the region would run out of new numbers by January.
"People are gobbling up numbers," Chow said.
To address the problem, the CPUC had two options: cut the geographic 408 area into two parts, forcing some residents to switch area codes, or assign the 669 area code to new numbers, meaning both area codes would exist in the same geographic area. The agency chose the latter option.
The downside is that all residents in the 408 region will now have to dial a "1" then the area code to make local calls. Those using cellphones won't have to dial the "1".
Phone companies will still charge the calls as local calls, Chow said.
The 408 area code has existed since 1959, according to the CPUC. Prior to that, the South Bay was part of the 415 area code until the region was split up because of growing demand for numbers, CPUC officials said.
The decision to add the 669 area code was made by the CPUC in October 2011.