Ps there is still the iridium paging service which is one way and is not transmit before receive. But they want to get rid of it and stopped selling hardware. It's really hard to find now second hand.
The only thing you can receive without transmitting is a one-bit "message waiting" indicator, so you don't waste uplink bandwidth polling the satellite unless there's something there for you. Basically the satellite broadcasts a list of terminals that have messages waiting for them. I recall there being some kind of bloom filter, so technically it's less than one bit.
You can't get anything else downstream unless you transmit first to ask for it. Also the only way to clear the "message waiting" bit is by transmitting to the satellite, so you can't even try to encode a message by flipping that bit on and off.
You had to manually update your paging location by dialing some number or by using a web interface when moving significantly, or link an Iridium phone to your pager, which would then automatically update the paging area (MDA) every time you made a call.
What you mean is probably Iridium SBD, but that’s a different, bidirectional service (also used for pager-like devices these days).
I did not know about Global Data Burst; I think that's what you're referring to. Holy cow it's expensive though, although I guess it has to be since you're using a lot more bandwidth*footprint. Like $6.00 per message per delivery area expensive. And they're kinda vague on how big a "delivery area" is.
GDB seems to be a successor to that, though.
Iridium Burst seems to be multicast-capable, and optionally bidirectional or unidirectional, so "spicier SBD" makes sense!
But their paging service was definitely unidirectional and unicast. Here's an old talk on decoding the signal, if you're curious: https://media.ccc.de/v/31c3_-_6236_-_en_-_saal_1_-_201412281...
I had one and I've only been outside the coverage area once, when I went on cave trip. I noticed my pager said OUTRANGE which was something I had never seen any other time. The coverage really was phenomenal due to the low frequency (159Mhz).