Following T-Mobile’s serious network outage back in June, FCC Chairman Ajit Pai pledged that he would investigate what caused the “unacceptable” outage. Today that report has come out.

Pai describes T-Mobile’s outage as a “failure”, saying that the FCC’s investigation found that T-Mo did not follow “several established network reliability best practices” that could have either prevented the outage completely or at least mitigated the impact it had on T-Mobile customers.

The outage was caused by an equipment failure and was exacerbated by a network routing misconfiguration, according to the FCC’s report. It goes on to say that the outage was magnified by a software flaw in T-Mobile’s network that had been latent for months and ended up interfering with T-Mobile customers’ ability to make or receive calls during the outage.

You can read the full FCC report here, which goes into much greater detail about what caused the outage and how it spread to become a nationwide issue.

The FCC estimates that during the outage which lasted more than 12 hours, at least 41% of all calls on T-Mobile’s network failed, including at least 23,621 failed calls to 911.

When asked about the FCC’s report, T-Mobile provided TmoNews with the following statement:

“We take our commitment to keep our customers connected seriously and our thousands of dedicated and passionate engineers are working to deliver for our customers every single day. Immediately following this incident back in June we took the necessary steps to address the issues that created the service interruption and remain committed to continual improvement.”

June’s outage was pretty serious for T-Mobile, starting in the southeast and spreading to the rest of the country. What’s more, it started in the middle of the day and lasted for the rest of the day, preventing people from making calls in the busy afternoon and evening hours. T-Mobile said in the days after the outage that it had added permanent safeguards to prevent this issue from happening again.

Source: FCC