Booking with d-reserve.jp

I’ve been trying to book a hotel in Niseko for January 2027, but I’ve been running into issues with the form through their direct website. It has the domain d-reserve.jp. I get an “invalid input” error, but it doesn’t tell me what is wrong. I think it might be an issue with my phone number. The page asks for a format +###-###-####-####, but I have a North American phone number with format +#-###-###-####. Both my wife and I have been trying and neither of us can get it to work.

Has anyone else had this issue? Any help would be appreciated!