A network outage is any event in which phone or data service stops working normally because of carrier equipment failure, software faults, overload, or a natural disaster. Now that mobile phones underpin daily life, a large-scale outage takes down far more than personal communication - payments, deliveries, and business systems go with it.
The best-known example in Japan is the KDDI (au) outage of July 2022. Voice calls and data service failed nationwide, and full recovery took several days. For the number of lines affected and the exact duration, consult the figures published by the carrier and the Ministry of Internal Affairs and Communications. Because emergency calls (110 and 119) were also disrupted, the outage made it plain that a network failure can put lives at risk.
Ways to prepare include keeping a line with a second carrier (a dual-SIM handset or a separate device), placing internet calls over Wi-Fi (LINE calls and similar apps), and using a public phone. Public phones run on the fixed-line network, so they are less exposed to a failure on the mobile side, and their coverage is guaranteed by the universal service framework. Note, however, that internet calls do not pass through the telephone network and cannot reach the 110 or 119 emergency numbers.
After the large-scale outages, the Ministry of Internal Affairs and Communications took up measures such as greater equipment redundancy, prompt notification of users while a failure is under way, and inter-carrier roaming so that emergency calls can be carried over another operator's network. The scope of these obligations and the operational details are subject to revision, so check the current position in material published by the Ministry and the carriers. See the disaster phone guide for alternatives to rely on during an outage.