NTT Docomo's mobile service has become difficult to use across Japan, the carrier said. Voice calls, data communications and SMS — short message service, the text messages sent between phones — are all affected.
Docomo said the disruption may also affect emergency calls. The carrier did not say what caused the problem or when service would return to normal.
The notice came as NHK reported separately that Docomo had suspended an online service over a possible unauthorized access. The carrier has not said whether the two are connected.
Japan's three big carriers each run their own nationwide networks, and subscribers generally cannot switch to a rival's signal when their own goes down. A single fault can therefore leave calls, payments and messaging unavailable to millions of people at once.