Boyne said the California Highway Patrol and other agencies in the region had similar problems with their 911 systems.
The Sheriff's Department contacted AT&T Inc., which fixed the problem, he said.
During the outage, callers were able to dial regular business numbers to reach dispatchers, and there were no reports of serious problems or emergency calls that went unanswered, Boyne said.
The outage occurred mainly in the 714 area code and was caused by Saturday's introduction of a new 657 area code, Boyne said.
AT&T spokesman John Britton said company officials were investigating why their system upgrade caused the outtage.
Beginning Saturday, people in the 714 area had to dial "1" and the area code to place a call from their homes phones, even to call people within the same area code. The change is a run-up to the scheduled Sept. 23 introduction of the 657 area code for new phones in the region.
The state Public Utilities Commission assigned the overlay code to deal with rising demand for phone numbers in the area.