@10G I have owned an Irridian for quite a few years, the reason I bought it (2nd hand) was the sim card in my post paid mobile phone worked in the sat phone as they were both on Telstra networks.
So when in need of comms away from normal service just swap the chip to the other phone and as long as it was set on roam we were sweet. Also we had to have a clear line of sight to the satelites, bit like a GPS between tall trees on a cloudy day the can take a while to latch on.
Telstra also did a $20 post paid card and sometimes we bought one of them for a bigger group and split the bill later, it went up to $650 on one trip as one woman had to talk to all her family every few days!
At the time Tharasa was just being introduced to Australia and only had one satellite available, bit hit and miss with signal and had to be outside the vehicle. Ron Moon did a story about them then outlining the limitations but as with all things technical it would have moved on from those days.
Today there is a clip on sat phone adaptor that fits to the back of your mobile, know nothing about it but sounds simple. Pat Kallanan was spruiking it on one of his TV shows.