Some NTT Docomo Inc. mobile phone customers were still facing disruptions to voice and data communications Friday, a day after Japan's largest mobile carrier reported a nationwide system outage.
A system issue occurred around 5 p.m. Thursday during work on the carrier's network, leaving 2 million people without voice and data services at one point. It said the system was restored three hours later but connections remained unstable for some users due to network congestion.
On Friday, the wireless carrier with some 80 million contracts 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.
"We apologize for the inconvenience and concern caused to many people," Hozumi Tamura, senior executive vice president of NTT Docomo, said at a press conference. He did not indicate when the network will be fully restored.
Internal Affairs and Communications Minister Yasushi Kaneko said Friday the ministry has asked NTT Docomo to investigate the cause of the massive service disruption 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, he said.
If it does, the carrier may be instructed to improve its operational response and ensure that a similar issue does not recur.
Low-cost carriers using NTT Docomo's network also suffered problems, such as phones showing no service and people not being able to use 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.
