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, however 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)

Presumably, the Second Doctor, seen assembling the components to summon the Time Lords about the War Lords, used a hybercube. (DW: The War Games)

Behind the scenes

 * In Real World science, a hypercube means a higher-dimensional analogue to a cube, the former have having four, the latter having three dimensions.