The Listeners were a special unit that existed within Starfleet Intelligence.
Made up of individuals from varied backgrounds, the group was the backbone of Starfleet's intelligence division. Its members included sleeper agents committed to a lifetime of covert service within an enemy government, such as Vulcans posing as Romulans, or Humans surgically altered to resemble other species; troubleshooting operatives thrown into critical, unfolding crises to aid in damage control; and communication officers on active duty aboard Starfleet vessels, who listened to everything.
Two examples of assignments handled by these personnel involved sudden trade agreements within sectors controlled by the Orions, which enabled pirates to smuggle arms, and intercepted rumors of the death by food poisoning of the Coridanite Ambassador in a carefully planned assassination.
In 2360, Admiral Nyota Uhura, the head of Starfleet Intelligence, employed the Listeners to confirm the presence of the disease known as the Gnawing. (TLE novel: Catalyst of Sorrows)