The government issued an administrative order Friday to NTT Docomo Inc. following a system failure last month that affected at least 12.9 million users and was labeled a "serious incident" by the communications ministry.
The ministry ordered Japan's largest mobile carrier to prevent a recurrence after many users nationwide were left without voice and data services at one point on Oct. 14 due to the failure caused by work on NTT's network. It took about 29 hours for services to be fully restored.
The ministry also urged the firm to improve ways to send information to users after the carrier's move to announce the restoration of service when some still had difficulty accessing the network led to confusion among users.
The law on telecommunications business stipulates that for a call service that handles emergency calls, a failure that affects more than 30,000 people for more than an hour is considered a serious incident.
The outage was triggered by a malfunction during work on its network of electronic payment equipment installed in such services as taxis and vending machines, according to the telecommunication giant.
NTT Docomo submitted an incident report to the Internal Affairs and Communications Ministry earlier this month that outlined the causes and preventive measures, such as introducing a new system to separately control mobile phone communications from other services when a system failure occurs.
The company also said that eight executives, including President and CEO Motoyuki Ii, will voluntarily return a portion of their salaries to take responsibility for the incident.