Some curious person had connected a lineman's set to a ringing phone line and noticed that, between the extremely loud ring signals, he could hear the calling party talking to himself. Also, that if he briefly shorted the ringing line, the ringing would stop but the call would stay connected. And he could now clearly hear the distant party! Moreover, if the calling party was using a pay phone, he could hang up and get his dime back, even if he had read the Declaration of Independence into the phone in the meantime. IOW, there was a functioning talk path but no billing!
So, the only problem to be solved was to get audio into the formerly ringing line without drawing enough DC from the line to look like a real phone call was in progress (and thereby trigger billing). This was easily solved by placing a capacitor in the line to keep the phone from drawing DC and placing a six-volt battery across the line, in series with an audio frequency choke. The final piece was a momentary contact switch (the clicker) placed across the line ahead of the capacitor. If you got a long distance call, you clicked the switch to stop the ringing, then picked up the phone, and talked for hours. And the calling party never saw the call on their bill.
If you wanted to initiate a call, you could "code ring" the party, signaling them to call you by prearrangement.
If memory serves, the Black Box stopped working when they rolled out ESS in the late seventies.