In Carnival's case, the call get beem up to a satalite and the satalite beems it to Miami than from there get routed to where the call need to go. This is pretty much why its expensive to use the service... but its alot less than the phone onboard. The only thing you have to watch out while in port is which network you are on and in most cases, international roaming applies. For me, I just have my phone off the whole trip because I personally going on a cruise to relax and enjoy the cruise, not chat on a phone all day.

As for the network itself onboard the ship created by Cingular / AT&T... customers with other cellphone carriers are able to use the network, with the exception of Verizon and one other company that refuse to sign on to the program with Cingular. According to some random cruise industry magazine I was reading on my last cruise.