Mobile phone service operator NTT Docomo said it confirmed that there was unauthorized external access on November 4 to its online shop, where users can purchase smartphones, and its “My docomo” portal, which allows customers to modify contract details.

To deal with the situation, the company began emergency maintenance at around 1 a.m. on November 5 and later confirmed that as many as around 300,000 customer information, including names, addresses, dates of birth and mobile phone numbers had been leaked.

Docomo said the cause and the scope of the impact are under investigation.

The firm is warning customers that leaked information could be misused, possibly leading to suspicious emails impersonating Docomo or related parties.

In a separate incident, starting around 11 a.m. on November 5, users nationwide had trouble accessing voice calls, data communications and SMS services on Docomo's high-performance 5G SA service, which operates on dedicated 5G infrastructure

The company said investigations so far have not found signs of unauthorized access or indications of a cyberattack concerning the communications disruption.

The 5G SA service was restored at 1:20 a.m. on November 6.

The disruption was caused by network congestion after data exceeding processing capacity concentrated on switches handling 5G SA subscribers.

Investigations so far have found no evidence of unauthorized access or any indication of a cyberattack before, during or after the incident.