Nippon Telegraph and Telephone Corp.'s regional subsidiaries said their internet and landline telephone services experienced disruptions Monday, affecting up to 446,000 internet lines across Japan.

The disruptions were caused by a failure in communications equipment on Monday morning, the first business day of fiscal 2023, before services were restored hours later, Nippon Telegraph and Telephone East Corp. and Nippon Telegraph and Telephone West Corp. said.

NTT Docomo Inc. and SoftBank Corp., which both use NTT's optical line services, also experienced connection issues following the disruptions at NTT.

At an online press conference, officials from NTT East and NTT West acknowledged the outage constituted a "serious incident" under the law.

File photo taken in March 2012 shows the headquarters of Nippon Telegraph and Telephone East Corp. in Tokyo. (Kyodo)

The communications ministry may issue administrative orders to both companies depending on the cause, which is currently under investigation.

NTT East confirmed service issues occurred around 7:10 a.m., affecting subscribers to its optical network service in Tokyo and the three neighboring prefectures of Kanagawa, Saitama and Chiba, as well as Niigata Prefecture and the northernmost prefecture of Hokkaido.

Up to 359,000 internet lines and 186,000 landline telephone services were affected.

Customers were temporarily unable to make emergency calls, and the companies requested that they use mobile or public phones instead. The services by NTT East were restored around 10 a.m., it said.

NTT West said similar disruption occurred in several areas, including Osaka, Ishikawa and Ehime prefectures. Up to 87,000 internet lines and 47,000 landline telephone services were affected, but they were able to fix the issue in around one and a half hours, the company said.

The disruptions follow a series of network-related issues occurring recently in the industry. Among other companies, mobile carriers KDDI Corp. and NTT Docomo experienced major service glitches in 2022.

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