Subspace communications allow starships, stations and planets to exchange messages faster than light, relying on transmitters and relay networks that span known space.
Radio signals would take years to cross interstellar distances, so starships transmit through subspace instead. Messages travel through networks of relay stations and boosters, and even then range is limited, which is why ships on the frontier can be out of contact with Starfleet for long periods.
Voyager's isolation in the Delta Quadrant showed how much depended on the system. The crew first made contact with Starfleet by using an ancient Hirogen relay network to transmit the Doctor to the Alpha Quadrant. Later, Reginald Barclay worked on the Pathfinder Project to open a regular communications link with the ship.