The US Federal Communications Commission (FCC) took the rare step of investigating a network service outage, after T-Mobile US customers lost voice and SMS access for more than 12 hours yesterday (15 June).

FCC chairman Ajit Pai revealed the action in a tweet, branding the problem “unacceptable” and stating the regulator and consumers were “demanding answers”.

The FCC typically only investigates severe failures. In June 2018, it fined AT&T $5.25 million for what were deemed preventable problems restricting access to the US 911 emergency service number.

In 2019, the regulator probed an outage at broadband provider Centurylink, but issued only recommendations to improve network reliability rather than a fine.

Pai did not say how far reaching the FCC’s investigation into T-Mobile would be, nor comment on potential disciplinary action.

In a statement, T-Mobile CEO Mike Sievert explained the problem was caused by an IP traffic-related issue which “created significant capacity issues in the network core throughout the day”.

Sievert said data services continued to work, meaning customers could make contact through services including FaceTime, iMessage, Google Duo, Zoom and Skype.