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Seems impossible to me that foreign operators are deliberately charged less... I think it's just a problem of disorganized inter-operators agreements (e.g. Riiing is not aware of the difference between Tim inlands and Tim onboard), but that's just my impression.

I found some information on how Gsm onboard ships works on
http://www.thedigitalship.com/powerpoints/...0altobridge.pdf.
It mentions that part of the price charged goes directly to the ship (who installed the equipment onboard...)

I believe that, of course, you might have already seen
http://www.geolink.com/oceancell.htm (quoting superferries - which, by the way, is not a greek company!).

Interesting: my understanding is that the ship network gets disconnected automatically, when reaching the costline (to avoid people inland to roam on the ship) through GPS... Amazing, isn't it?