In the earlier days, quite a number of the HF links were just regular double-sideband AM with carrier, not even SSB. Even today we still have ship-to-shore telephone links operated over simple, unencrypted SSB links in the marine band (2 to 3MHz-ish). I'm on the east coast of England, and there are still quite a number of calls placed this way from the off-shore oil rigs in the North Sea.
When satellites started taking over international circuits, there was no encryption either. They used the same sort of basic carrier arrangements as were already in use for coax systems, with each circuit in an SSB channel of 4kHz overall bandwidth (wider than the telephone speech bandwidth to allow for guard bands, pilot tones, etc.).
12 channels made up a group, then 5 groups were multiplexed together to form a supergroup, and so on. The arrangement allowed for easy routing of circuits at intermediate points without splitting everything down to individual channels and then recombining. If, say, one group (12 channels) out of a supergroup was destined for a particular place,you could just extract that group complete and then multiplex it on to a different supergroup to send it on its way -- No need to worry about the individual channel content of the group.Anyone with a suitable satellite receiver/downconverter could connect it to a receiver and then just tune across the band, finding an SSB telephone channel every 4kHz or so. In fact when I worked for BT in the 1980s that's almost EXACTLY the way we would check a specific channel off the satellite (or other carrier systems). We had monitor sets that we could connect to the downconverted satellite baseband. They had precisely calibrated level meters so we could measure pilot-tone levels to make adjustments, but they were still, in effect, just fancy SSB SW receivers.
Of course, in this case you'd be picking up just one direction of the conversation, as hybrids were used back at the origin to split the circuit. Finding the "mate" of a channel was easy enough with all the records in front of you, although even that could take a while to set up. For someone tuning around without that information, it would be much, much harder.
Paul