Booking of Zamani Queen Ferry

I am thinking of booking return tickets via telephone, but I do not speak Japanese (just simple words like dates and time is fine for me though)

Would it be easy for me to book via telephone?

Also, after booking of tickets, would I be able to change the return leg if I need more time on the island? How should I go about doing that? Can hotel / home-stay owner call ferry operator and do it via telephone (if they are willing to do it)?

According to the web site below, indeed, the operator of the ferry does not seem good at English, but if it is possible to convey to them the intention to be on board, you might be capable of it.
http://www.zamamienglishguide.com/ferries.html