The company must also put a compliance plan in place that contains new commitments by T-Mobile to improve the 911 outage notices given to call centers, or public safety answering points, the FCC said in a consent decree.
This includes providing call centers with information about the outages and follow-up notices within two hours of the initial outage notifications.
"We understand how critical reliable connectivity is to ensure public safety and we take that responsibility very seriously," T-Mobile, the No. 3 mobile service provider in the U.S., said a statement.
"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 complaint dates back to June 15, 2020, when the outage, which began at 12:33 p.m. Eastern, led to congestion across T-Mobile’s 4G, 3G and 2G networks, and caused "the complete failure of more than 23,000 911 calls," the FCC said.
There were also calls that did go through but without important information, like a callback number or location data.
At 2:41 p.m., T-Mobile initiated mass 911 outage notifications to the thousands of potentially affected call centers, which warned that delivery of location information may be affected. The notifications also stated that “911 calls are still completing.”
After the outage was resolved, T-Mobile sent a final notification. The initial cause of the outage was the brief failure of a leased fiber transport link in the T-Mobile network.
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 hampered by a temporary failure of remote access to the affected transport link, the FCC said.
T-Mobile paid a $17.5 million fine after two related nationwide service outages on the same day in August 2014.
In August of this year, T-Mobile said a total 13.3 million active customers were affected by a data breach and more than 40 million records of former and prospective customers were also accessed in the hack.
Shares of the Bellevue, Wash., company were down 1.2% to $114.10 at last check.