An international telephone number, International Code or state key, is a sequence consisting of one, two or three decimal digits (0-9); it is used when communicating between states, so that each state identifies a special number; when calling a state - the caller dials the number of the state concerned, followed by the area number and then the target telephone number; the dialing process requires the international line to be opened first - this varies from state to state, although in 33 states it is common for two consecutive zero: 00 to be dialed first.