The telegraph was one of the first form of communications technology developed on Earth, during the 19th century.
The Excalbian recreation of Abraham Lincoln referred to Uhura's communications system as a "voice telegraph device." (TOS: "The Savage Curtain")
Worf used components of a telegraph, in combination with his communicator, to create a personal deflector shield against projectiles in the damaged holodeck simulation of Deadwood. (TNG: "A Fistful of Datas")
While a salvage team lead by Miles O'Brien was stranded on Empok Nor, it was suggested that if the field coils in the deflector grid were modified to emit a series of covariant pulses, while the power was boosted to the induction grid, they could use the station "like an old-fashioned telegraph and tap out an SOS" to send to Deep Space 9. (DS9: "Empok Nor")