AS I posted in the thread below, I was charged exactly the US rate listed on their website when I used my IOM sim. Carriers have specific rates listed on their websites for cruise ship roaming. But with these prepaid sims like UM+, yackie, etc. may be it depends on the country provider of the service is registered in. In my case it was provided by WMS which I think is US based.
http://www.prepaidgsm.net/forum/crui...ing-t3159.html
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile