A phase change is a action made by the server to switch between time sections that constitutes a game; the server will do all the calculations during those events wich will also be used to synchronize all the players in the game.
Those phase changes occurs when:
After only one or less player is alive in the map and all the shoots were displayed, the game will perform this action and will provide points and money and interests to the players according the server settings; this can also be triggered after a No# of shots has been made in this turn (this value is being settable in the server too).
After the No# of turns that the game allows expires (settable by the server) or the number of current players drops below the minumum players allowed value the server will trigger this action.