Dictagraph
The dictagraph was a 20th century device used for covertly recording the activities in a room. The device was connected by telephone wires to a dictagraph plate which picked up noises in the room. Using receivers, individuals could hear what was happening. Stenographers would write down what was heard through the receivers. 

This device was mentioned in a detective novel Data was reading in early 2364 to become more prepared for a Dixon Hill adventure. ()

Communications technology