• Kyodo, Jiji

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Some NTT Docomo Inc. mobile phone customers continued to have difficulty making calls and data connections on Friday, a day after Japan's largest mobile carrier was hit by a nationwide system outage.

NTT Docomo said it suffered a system issue at around 5 p.m. Thursday during work on its network. Services were restored three hours later but connections remained unstable for some users due to network congestion, the carrier said.

The wireless carrier with some 80 million contracts said Friday it limited data traffic following the network congestion.

The nationwide disruption of its services has affected some 2 million users, the firm said.

The affected customers could not connect to NTT Docomo's networks, being unable to register their location information necessary for the services, according to the company. The number of affected people may increase if those who had a minor inconvenience in making or receiving phone calls are included.

Major 4G and next-generation 5G networks returned to normal at 5:05 a.m. But the 3G network used by older pre-smartphone devices had yet to be fixed.

"We apologize for the inconvenience and concern caused to many people," Hozumi Tamura, senior executive vice president of NTT Docomo, said at a news conference on Friday afternoon. He did not indicate when the network will be fully restored.

Low-cost carriers using NTT Docomo's network also suffered problems, such as phones showing they are without service and users not being able to send text messages.

"It is very regrettable that such a large-scale disruption has occurred for mobile phone services, a key infrastructure for people's lives," communications minister Yasushi Kaneko told a news conference Friday.

Kaneko said that the communications ministry has instructed NTT Docomo to look into the cause of the problem and submit a report quickly. Based on the report, the ministry will determine whether the disruption amounts to a serious incident under the telecommunications business law, he added.

If it does, the carrier may be instructed to improve its operational response and ensure that a similar issue does not happen again.

In January 2019, the communications ministry instructed SoftBank Corp., Japan's third-largest carrier by user base, to improve its response to a similar issue after faulty software provided by Telefonaktiebolaget LM Ericsson caused an outage that affected around 30.6 million subscriber accounts for over 4 hours.

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