The Federal Communications Commission (FCC) slammed T-Mobile US for failing to follow network reliability best practices, stating in a report the operator could have prevented a massive service outage in June.

Its probe into the more than 12-hour disruption determined it was initially caused by an equipment failure, which was compounded by a network misconfiguration related to the introduction of routing equipment and a previously undiscovered software error.

The FCC estimated at least 41 per cent of all calls on T-Mobile’s network failed during the outage, including more than 23,000 calls to the US 911 emergency service number.

It concluded T-Mobile could have prevented or lessened the impact of the outage by auditing its network while integrating the new router equipment and more thorough lab testing of the software prior to installation. It added the operator had since taken remedial action.

In a statement, FCC chairman Ajit Pai called the incident “a failure”, adding “all telecommunications providers must ensure they are adhering to relevant industry best practices”.

The FCC fined AT&T $5.25 million in 2018 after determining preventable issues caused two network outages which prevented thousands of 911 calls.