Why is Iridium connection cut so often when I call? Why Iridium in only solution for difficult locations?
I hope this gives some explanation:
Thuraya and Inmarsat use geostationary satellites - they stay in the same direction all the time. If phone can "see" the satellite it will not disappear. But if satellite is not seen - behind the horizon or behind building, hill or mountain - then it is out of sight and there it stays. Only thing which helps is to move location.
Instead of these two, Iridium use low orbit satellites - they are like high flying airplanes which pass the sky. Sometimes satellite is in low in northern sky, sometimes above you, sometimes middle high at western sky, etc. Sometimes you see only one, sometimes several. Iridium network has been designed so that in open place phone will see always one or more satellites - more near poles - less at equator. Even when you are between mountains there will be, sooner or later, a satellite passing passing above you so that phone will see a satellite.
Less you see the open sky between obstacles, longer can be the waiting time and shorter is the time to place a call, but there is satellite available for call sooner or later. There is on-line tracking site where you can track also Iridium satellites and see how they pass your area. Just select Select all and then Track selected satellites. It gives good visual presentation of operation of Iridium satellite network and hopefully explains more questions set above.