The Listeners were a group that existed within Starfleet Intelligence.
They were known to haven been a group of many individuals of varied backgrounds that were the backbone of Starfleet's intelligence division. These individuals ranged from sleepers who had committed themselves to a lifetime within an enemy government such as Vulcans posing as Romulans to Humans who were surgically altered to resemble a dozen other species that bore watching as well as troubleshooters who were thrown into crises whilst they were happening and attempted to aid in damage control. There were also communication officers on dozens of Starfleet vessels that were active throughout known space who listened to everything.
A coupe examples of duties among these personnel covered sudden trade agreements from Orion controlled sectors which meant pirates were smuggling arms and rumors of Coridanite Ambassadors death by food poisoning through a carefully planned assassination.
In 2360, Admiral Nyota Uhura who was the head of Starfleet Intelligence made use of the Listeners to get a confirmation on the presence of the disease known as the Gnawing. (TLE novel: Catalyst of Sorrows)