Classicly

The Emergency Call had been instituted over two thousand years ago, after a disastrous invasion of the system by beings from some unknown part of space. Installed in all important etheradio stations, it was to be used only in cases of extreme urgency, and at its call all government officials were to drop everything and repair to the nearest etheradio for instructions.

In the two thousand years it had been in effect it had sounded only three times, apart from the regular tests: once when a giant meteor had threatened the system with destruction, once when the unknown invaders had attempted a second attack, and once when miners on Mira had drilled into an inner world inhabited by creatures with hypnotic powers that threatened to enslave the planet.