Parallel dimension (Telepresence)

During the Last Great Time War, crashed his TARDIS in a parallel dimension while "exploring possibilities". Sometime after, the Cybermen of the Cyber-Mainframe broke into this dimension as part of their campaign to upgrade the entire multiverse. The Master was able to avoid being detected by the Cybermen and adapted numerous salvaged pieces of Cyber-Technology to create several handheld weapons, in addition to repairing his TARDIS. When the Cybermen created a dimension bridge to N-Space, the Master flew his TARDIS through the bridge and back into his home dimension, and begrudgingly assisted Kate Stewart in stopping the Cybermen's conversion of the Earth of N-Space. Shortly after the Master left in his TARDIS to return to the Last Great Time War, the dimension bridge connecting N-Space to this dimension shut down. (AUDIO: Master of Worlds)

Behind the scenes
Despite the similarities between the two locations, it is never stated in AUDIO: Master of Worlds or in the other stories of the boxset UNIT: Cyber-Reality if the parallel dimension that was trapped in is the dimension as that of the parallel Earth explored by Osgood, Shindi and Josh in AUDIO: Telepresence.