For six hours, nearly 750,000 California residents would not have been able to reach an emergency call center if they dialed 911 on their mobile phones.
Those six hours will cost Verizon $3.4 million.
A problem in a Verizon-run 911 system resulted in the outage, which affected nine counties in California. Verizon is required by law to report outages within 30 minutes of a problem, but officials didn't hear about the April outage until it was resolved.
After an investigation, the Federal Communications Commission reached a settlement with Verizon designed to improve 911-call-center reliability and the outage-reporting process. Verizon was also fined for the oversight.
According to the investigation, Verizon claims the outage was caused by a malfunction in a subcontractor's infrastructure. The subcontractor didn't notify Verizon until the outage was resolved, which is why Verizon didn't notify officials, the company said. But since Verizon is responsible for its contractors, it's on the hook for the fine and the conditions of the settlement.
"We take the safety of our customers and the service we provide to 911 centers and first responders very seriously," a Verizon spokesperson said in a statement. "We will continue to work with our partners to meet the standards our customers expect from Verizon."
Verizon is conducting a trial in California of a 911 service that captures location data from wireless callers, a standard called Enhanced 911 which the FCC set in 1996. Technical limitations have slowed the rollout of Enhanced 911, which is slated for full implementation in 2019.
"Americans must have confidence that they will be able to reach 911 in an emergency," said FCC Chairman Tom Wheeler in a statement. "We take seriously our obligation to ensure the nation's 911 systems function reliably. We will continue to work with providers to ensure that advances in 911 technologies lead to improved communications between citizens and first responders."
Funding for 911 services comes in large part from federal fees on consumers' phone bills. According to the National Emergency Number Association, as of 2014, California residents paid three-fourths of 1 percent of their bills for 911 services.
The outage in California was part of a larger technical problem that affected seven states and more than 11 million people, according to the FCC.
This article was updated to include a statement from Verizon.