Back in the mid-90s, I actually bumped into one before I knew about their existence. One night while slowly scanning the shortwave dial, suddenly there was this female voice reciting strings of numbers in an almost robotic way. Then it stopped, and switched to the static of a frequency that is transmitting no signal.
As it turns out, these things have been going on for decades, there have been hundreds of them, and nobody in the general public knows:
What they are saying, as the numbers are surely a coded message.
Who and where is sending them.
Who and where are they being sent to.
The most probable explanation is espionage, but nobody really knows.