Radio silence is an operating protocol for starships that are either in combat or operating in a combat zone. When radio silence is invoked, the ship sends absolutely no outgoing communications – not even to acknowledge incoming transmissions.
While it was stranded behind enemy lines in early 2374, the USS Valiant observed strict radio silence to avoid detection by the Jem'Hadar. This had the unfortunate side effect of leaving Starfleet Command unaware of the fact that the Valiant was being operated solely by the surviving cadet members of Red Squad. (DS9: "Valiant")