The operator is using equipment from Ruckus Wireless and has already installed 10,000 access points. It is able to install the access points quickly in part because it is using WiMax for the back-haul. By mid-2012, it expects to have more than 100,000 access points installed, according to Ruckus, which plans to announce the project Tuesday.

As more mobile customers use smartphones and applications that consume data, mobile networks are struggling to keep up. Operators are considering a number of options for boosting capacity and coverage, including small cellular base stations called femtocells, and Wi-Fi.

Subscribers to KDDI's flat-rate plans will automatically be attached to one of the Wi-Fi hotspots rather than use the cellular network, Ruckus said. Customers will need special software on their phones. Existing customers can get the software by visiting a KDDI web site from their phone. New customers will receive phones with the software preloaded.

Once customers have the software, "the phone automatically recognizes and logs on to any available access point," said Steve Martin, vice president of engineering for Ruckus.

For now, the software, which KDDI developed with Ruckus, works only on Android phones. But the operator plans to enable the service on other phone platforms too, Ruckus said.