Japan's answer to Amazon, Rakuten, just got into the mobile communications business by purchasing internet calling app-maker Viber for $900 million. Viber was one of the first to introduce 3G VoIP on the iPhone, and now lets you call any land-line or mobile phone, send messages and even push-to-talk. The app' available on all major mobile and desktop platforms, and is one of Skype's biggest rivals with over 300 million users worldwide. According to the WSJ, Viber's global userbase helped draw Rakuten, which is trying to expand its business interests beyond Japan's shores. It also recently purchased a huge stake in Pinterest and bought e-reader maker Kobo. Rakuten's CEO confirmed that Viber "perfectly compliments" its goals and said the acquisition "will enable it to penetrate new markets with multiple digital content offerings."