Unless stated otherwise, all information herein is taken from ACoS, Ch. 20.
A callbox is a ter'angreal that can be used to communicate over large distances. Sammael gives one to Sevanna, in the form of a small grey cube, apparently of stone, yet much harder. It had intricate patterns covering every side, with smaller patterns within, and smaller still, until the eye could no longer make them out. It was activated by touching with a small weave of fire, on two markings, a crescent moon and a lightning. When this is done, it draws The Power on its own, both saidar and saidin