WASHINGTON (Reuters) - The Federal Communications Commission (FCC) said a June nationwide T-Mobile outage resulted in at least 250 million calls failing. FCC chairman Ajit Pai said a staff report released on Thursday found the company did not follow several established network reliability best practices that could have potentially prevented or mitigated the outage. “T-Mobile’s outage was a failure,” Pai said. The June 15 outage on T-Mobile’s U.S. wireless networks lasted over 12 hours, disrupting calling and texting services nationwide and access to data service in some areas, the FCC said, adding at least 41% of calls on T-Mobile’s network failed during the outage. T-Mobile said it takes its “commitment to keep our customers connected seriously” and added it took “necessary steps to address the issues that created the service interruption.” The FCC staff report said the outage was caused “by an equipment failure and then exacerbated by a network routing misconfiguration that occurred when T-Mobile introduced a new router into its network.” The FCC said the “outage was magnified by a software flaw in T-Mobile’s network that had been latent for months and interfered with customers’ ability to initiate or receive voice calls during the outage.” The FCC staff report said there were at least 23,621 failed emergency calls to 911 as a result. T-Mobile said its network experienced an 18% reduction in completed calls during the outage. The FCC said the figure did not address the full problem. “T-Mobile acknowledges that network congestion likely required many of its subscribers to make 2-3 call attempts before successfully connecting,” the report said. The FCC estimated “that over 250 million calls ... from other service providers’ subscribers to T-Mobile subscribers failed due to the outage” and “at least 41% of all calls that attempted to use T-Mobile’s network during the outage did not complete successfully.” T-Mobile in April officially completed its $23 billion merger with Sprint, solidifying its position as the No.3 wireless provider in the United States. The FCC said legacy Sprint customers were unaffected by the outage.