Nippon Telegraph and Telephone's regional subsidiaries said their internet and landline telephone services experienced disruptions across Japan on Monday.
The companies said their services were restored hours after problems began on Monday morning, the first business day of the 2023 fiscal year.
NTT Docomo and SoftBank, which both use NTT's optical line services, also experienced connection issues following the disruptions at NTT.
The communications ministry said it had dispatched officials to NTT East and NTT West to identify what caused the disruptions.
NTT East confirmed service issues occurred around 7:10 a.m., affecting subscribers to its optical network service in Tokyo, Kanagawa Prefecture and Hokkaido.
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. They were able to fix the issue after about 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 and NTT Docomo experienced major service glitches in 2022.