KDDI and SoftBank are teaming up to launch a new service that allows subscribers to connect their smartphones to each other's mobile networks by using a back-up SIM card in case one of them faces a large-scale outage.

The announcement on Thursday is a response to KDDI’s massive mobile network outage last summer that affected more than 30 million users and other services such as ATMs. To deal with the problem, the government and mobile operators have launched discussions on ways to ensure users can stay connected in times of emergency.

Japan’s second and third largest mobile operators said they will roll out the dual-SIM option in late March, a rare initiative for the nation’s top carriers.

KDDI and SoftBank said that they both recognize the importance of securing alternative means for users to access the internet or make calls through smartphones when the carriers are faced with network disruptions.

“While it’s a matter of course that we must not allow (the network to be disrupted) again, the biggest thing we realized was that there weren't really any alternative (means of communication available) when it happened,” said KDDI chief Makoto Takahashi during a news conference to disclose the firm’s quarterly earnings on Thursday.

In the past, pay phones and fixed home phones were more widely used, but now many people just rely on smartphones to contact others, Takahashi said.

The details of the dual-SIM service are yet to be disclosed, but Takahashi said KDDI is thinking of charging “several hundred yen” as a basic monthly fee that will operate under a pay-as-you-go system, which is used only when something happens, adding that it will be similar to insurance.

KDDI is also in talks with NTT Docomo, Japan's largest carrier, but it is unclear when they will be able to provide a dual-SIM service, Takahashi said.

Having two SIM cards from the major carriers on one phone is already possible if users have dual-SIM-ready handsets, but they must be set up by the users themselves.

When asked why the top carriers did not provide such a service before, Takahashi said that “it was a common practice for carriers to take care of everything on their own (instead of cooperating with other carriers).

“But I think it’s important from now on to work together on things we can agree on,” such as preparing for unexpected problems, in order to save costs and promote healthy competition to provide better services, he added.

KDDI’s network failure occurred on July 2 last year as the firm was changing a setting on a router for its core network, part of its regular maintenance. The network disruption was the result of maintenance workers installing the incorrect setting.

While trying to fix the glitch, the carrier experienced heavy concentrations of traffic in parts of the network, prolonging the outage. It took KDDI more than three days to announce the full restoration of the network.

The carrier said about 22.8 million subscribers had experienced difficulties making calls, while more than 7.65 million users had trouble connecting to the internet during the outage.

The outage also affected business activities ranging from ATMs to parcel deliveries and weather data systems.

Given the gravity of the situation, the communications ministry launched a panel last September to discuss how the major mobile phone operators can provide cross-carrier emergency roaming services to prepare for when network troubles occur.

Communications minister Takeaki Matsumoto welcomed the new backup service by KDDI and SoftBank.

“I believe providing various services through initiatives like this will benefit users,” Matsumoto said.

"I hope efforts by business operators and related organizations to ensure that communications remain secure during emergencies, such as by providing a dual SIM service, will progress further."