It must be a truly harrowing experience if you have to contact first responders and the 911 call doesn’t connect, but that’s exactly what happened to thousands of T-Mobile customers in the U.S. during a 12-hour period on June 15, 2020.

As a result of the alarming incident, the wireless carrier this week agreed to pay $19.5 million in a settlement with the Federal Communications Commission (FCC).

According to the FCC’s report, last year’s nationwide 911 outage resulted in the “complete failure” of more than 23,000 emergency calls.

The initial cause of the downtime was attributed to the “brief failure of a leased fiber transport link in the T-Mobile network,” the FCC said, adding: “The outage revealed, and was compounded by, a temporary routing flaw in a single location and two previously undetected flaws in third-party software. Restoration was also impacted by a temporary failure of remote access to the affected transport link.”

In a widely reported statement on this week’s settlement with the FCC, T-Mobile said it had since improved the reliability of its emergency systems to ensure that 911 is available to customers when needed.

“We understand how critical reliable connectivity is to ensure public safety and we take that responsibility very seriously,” T-Mobile said in the statement. “We have built resiliency into our emergency systems to ensure that our 911 elements are available when they’re needed. This was a short-term isolated outage and we immediately took steps to further enhance our network to prevent this type of event from happening in the future.”

The company added that it was now “moving on from the FCC’s investigation and continuing our focus on our ongoing network build.”

It’s not the first time T-Mobile customers have suffered issues with calls to 911. Several outages in 2014 blocked the service for a total of three hours. The carrier agreed to pay the FCC $17.5 million to settle the case.