T-Mobile Blames Network Failure on a Fiber-Circuit Supplier

T-Mobile US Inc. said its nationwide service outage Monday stemmed from the failure of a Southeast fiber circuit that was leased from a wholesale network supplier, which triggered an overload and a “traffic storm.”

As many as 68 million of T-Mobile’s wireless customers had their mobile connections disrupted for several hours as the company tried to identify the problem and restore service.

With the help of hundreds of engineers and technicians, T-Mobile traced the root cause and fully restored service by 1 a.m. New York time on Tuesday, the carrier said. T-Mobile didn’t name the fiber-circuit supplier.

The company’s technology ...