"Our sole focus has been restoring full services for all customers; we are now investigating the root cause of the incident," it said.

The outage, which began around 5:30 p.m. on Tuesday, disrupted voice and some data services for 5 percent of T-Mobile's customers in various areas. Based on the 33.5 million subscribers that T-Mobile reported at the end of the second quarter, the outage affected nearly 1.7 million people.