Willcom
A smaller player in the market. Willcom took over the PHS business from docomo in 2008.
Willcom seems still more geared towards businesses who can use PHS communications internally as a way to cut costs. It should be also noted that PHS connectivity in Tokyo Metro is quite good. And until the appearance of HSDPA and emobile, their handsets boasted the fastest packet speeds (128Kbps).
Documentation for developers (only Japanese)
