Hi Ukap. Till I know the system in the ship is a special roaming agreement with few companies, like Cingular, Manx Telecom, and TIM Italy (in those websites you find it) controled by the operator MCP (Maritime Communication Partner) the say everything and only in some cruise.
The Ship has an MCP Erb that is turn on when it is far of the seaport. Those Erb make calls by satellity, that is why it is so expensive. Usually you pay the $2.99 to make or receive call. I will be very suprised if any of this prepaid system has agreement with those people.
What shows in the display is very different from operator to operator, in the MCP could apears MCP or 901-12, in the Manx will be 310730or Seamobile. So don't follow your display.
Check the website
www.mcp.com
Good luck and let me know if it works, because I will be surprised but happy.