The costing of the connection is a minefield. Some providers are cheaper for data transfer, others cheaper for voice calls. Not even the people who sell the phones can give you a straight answer (because they can't work it out either).
Like Stingo, I'm with Mailasail. For the money you also get a web diary. A bit like a blog except that people can't add comments! It's probably cheaper to pay up front for airtime, the more you buy the cheaper it gets. My calls work out at 80p per minute. Cheaper to use my mobile with it's international SIM card except you don't always have coverage (and strangely enough no coverage at all mid-Atlantic/Pacific

Got the phone at last years Sou boat Show. Brand new, data kit, all wiring etc etc, ready to connect up.....Ł1250. It's a 9505A. Iridium network is the way to go.