Call letters
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 computer's database a listing of the call letters of most registered ships and planets. Ships operating in neutral space that used call letters weren't always known by name. When a ship was in distress, it was customary for that ship to include its call letters in the distress call. When a ship was acting friendly, it was customary for it to transmit its call sign. An official organization assigned a unique call sign to each ship. 

In 2254, the  was able to identify a radio-interference distress call as coming from the  based on that ship's call letters. ()

In 2256, the shuttle DSC 01 had the call sign "Disco-1". ()

In 2265, the Enterprise was able to identify a distress call as coming from the  based on that ship's call letters. ()

In 2364, the  received no call letters from the  as the cruiser approached the larger ship. () 

In 2365, the Enterprise-D received the call sign of the Erstwhile as the cargo ship approached the larger ship. ()

A call sign was 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 a private transmitter. ()

Call letters were also used as a radio identifier for a planet. 

In 2366, the Enterprise-D didn't receive the call sign of the Federation colony on Rana IV. The planet had been devastated by a Husnock attack. ()

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Communications technology