Subspace was a dimension distinct from normal space.
Communication
Sonic subspace transmissions temporarily interfered with the Second Doctor's new sonic screwdriver. (PROSE: Dreams of Empire)
The Adamantium responded to a subspace distress call sent by Bunny Cheung from the Akoshemon system. (PROSE: Fear of the Dark)
After Gorsk's mission in Mega City was completed, he used a subspace radio to request pickup from the planet. (PROSE: Shakedown)
Strax reported to his fellow Sontarans that there was news on all subspace channels of an upcoming new incarnation of the Doctor. (PROSE: Deep Breath)
River Song modified her vortex manipulator to achieve subspace communication and used it to contact the Galactic Heritage. (AUDIO: The Sonomancer)
Other references
Strange icarons had no harmful effects on organic lifeforms, but interfered with starship jump engines. If a ship entered or exited subspace in an area flooded with strange icarons, their engine would explode. (PROSE: GodEngine) Conversely, normal icarons "preferred" to be in hyperspace and caused paranoia and psychotic behavior in carbon-based lifeforms. (PROSE: Original Sin)
The Charing Cross had a subspace buoy. (PROSE: Fear Itself)
Monica Lamprey used Joseph Tungard to create a subspace rift. (PROSE: Spiral Scratch)
The hull of an Osirian WarScarab was nearly unbreakable and opaque to subspace fields. (PROSE: GodEngine)