This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Call letters, or a call sign, was a radio identifier used for identifying a starship. This identifier was an alphanumeric sequence. Ships carried in their library computers a database listing the call letters of most registered ships and planets. Ships operating in neutral space who used call letters weren't always known by name. When a ship was in distress, it was customarily for that ship to include its call letters in the distress call. When a ship was acting friendly, it was customarily for it to transmit its call sign. An official organization assigned a unique call letter to each ship.
Call letters were a radio identifier used for a private transmitter.
In 2267, Captain James T. Kirk ordered his communications officer Nyota Uhura to contact his brother George Samuel Kirk and his family on the planet Deneva. Uhura recognized the alphanumeric sequence he gave (GSK-783) as being a call sign assigned to one of these transmitters. (TOS: "Operation -- Annihilate!")
Call letters were also used as a radio identifier for a planet.