On Jan 25, 2012, a major network outage at NTT Docomo Inc in Tokyo left 2.5 million subscribers unable to make voice or data calls for approximately four and a half hours.

The operator was in the process of upgrading its network to cope with the increase in signaling traffic from smartphones. But the new system was unable to cope with the volume of traffic, which led to a service outage starting at about 8:30 am and lasting until about 1 pm. Later, the company admitted it had underestimated the surge in data traffic from smartphone users.

The service collapse coincided with a major disruption of rail services that brought Tokyo's three major commuter lines and other rail lines to a halt for about an hour. The accident triggered a flood of calls and e-mails from delayed passengers.