Internal Affairs and Communications Minister Yasushi Kaneko holds a press conference in Tokyo on Oct. 15, 2021. (Kyodo)

       TOKYO (Kyodo) -- Some NTT Docomo Inc. mobile phone customers on Friday continued to face difficulties in making calls and data connections, 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. on Thursday due to troubles during work on its network. Services were restored three hours later, but connections remained unstable for some users due to network congestion as many people tried to access the network, the carrier said.

       Japan's largest wireless carrier with some 80 million contracts said Friday it limited data traffic following the network congestion. 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 was yet to be fixed.

       Internal Affairs and Communications Minister Yasushi Kaneko said Friday the ministry has requested NTT Docomo investigate the cause of the massive service disruption in detail and report the scope of users affected.

       "It is very regrettable that the (system) failure occurred to significantly affect the mobile phone service, important infrastructure for people's lives," Kaneko told reporters.

       The ministry intends to confirm whether the problem falls under "a serious incident" stipulated in the law on telecommunications business based on the carrier's report, Kaneko said.

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

       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 SMS.

       NTT Docomo said its system responded in an unexpected way during the work. Data that had backed up during the outage overwhelmed the system when the company tried to restore services, it added.

       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 about four and a half hours.

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