I'm trying to book a roundtrip ticket for the jetfoil ferry from Kagoshima to Yakushima that goes from Kagoshima to Miyanoura, and then the return leg goes from Anbo to Kagoshima. Looking at https://www.tykousoku.jp/ I can see that that is indeed an option, and return tickets are either JPY 22,300 or JPY 17,100 if the departure date is more than 2 weeks out. However, I'm unsure how to use the website for the full booking procedure; for example, registration requires a landline phone number.
On the web, I have found links to DirectFerries.com as a way to book tickets on this route. However, on DirectFerries if I search for roundtrip tickets, this route shows up as unavailable: the only options are to either book roundtrip from Kagoshima to Miyanoura and back, or Kagoshima to Anbo and back, at JPY 22,300. Regardless of the departure date, I can't find the JPY 17,100 price. To get the route I am looking for on DirectFerries, I need to book both as one-way tickets, for a total of JPY 24,400.
How can I book the return ticket from Kagoshima to Miyanoura and from Anbo to Kagoshima on either DirectFerries, or some other website, from outside Japan?