The most common input method on Japanese smartphones is also a 12-key keyboard modeled after feature phones, since it has the lucky coincidence that it maps onto the Japanese phonetic system perfectly - あかさたなはまやらわ+modifiers. On feature phones you had to multi-tap to get from あ to い to う, but on a touchscreen they added "flicking" into different directions to jump directly to a character. Once you get used to it, you can get really fast at it!

Animated demonstration: https://media.giphy.com/media/gjOWCOlhd98wI4dYcd/giphy.gif