Hypercube

Hypercubes were a type of communication technology used by the Time Lords which resembled cubes; they were used by thinking or speaking to them, and were then sent through time. They could be used by humans as well as Time Lords. (EDA: Vampire Science, The Ancestor Cell) They were theoretically impossible to intercept, but an agent of the Faction Paradox was able to intercept a cube the Eighth Doctor sent to the Time Lords. They were mainly owned by members of official Gallifreyan organisations. (EDA: Unnatural History)

The First Doctor collected his when he worked for the Bureau of Possible Events, but left them behind on Gallifrey when he escaped with Susan. However, it was returned to him by his mentor K'anpo Rimpoche, shortly before his first regeneration along with his Five Hundred Year Diary. (ST: The Three Paths)

The Second Doctor used a hypercube to ask the Timelords to return the soldiers kidnapped by the War Lords to their own time periods.(DW: The War Games)

Hundreds of Timelords, who were stranded in the bubble universe by House used hypercubes to send distress calls. The Corsair's hypercube arrived in the main universe and found the Doctor's TARDIS. After the Doctor arrived in the bubble universe, he heard the distress calls on Nephew's translation sphere and assumed that this meant that there were living Timelords. Unfortunately, all the Doctor found was a cabinet filled with hypercubes, as House had murdered all the Timelords.(DW: The Doctor's Wife)

Behind the scenes

 * In real-world science, a hypercube means a higher-dimensional analogue to a cube, the former having four dimensions, the latter only having three.