l. A busy signal is a call in the DIALING state that does not connect. 16.2.2 Advanced PSTN call states In Fig. 16.2 the only meaningful stimuli in the TALKING state are HANG-UP and THEY- HANG-UP. Somebody else trying to call gets a busy signal, and the current call status remains unchanged. Call waiting is a custom calling feature that allows an incoming call to interrupt a call in progress. A subscriber with this feature hears a tone during a call to signify a second caller. The second caller has the usual ringing sound, no signal that the call at- tempt is interrupting anything, The subscriber with call waiting can switch between the two callers using a flash (also called a switchhook flash). On a regular, plain-Jane telephone, a flash is executed by holding the switchhook down for about half a second, not long enough to end the call. Some fancy telephones have a special FLASH key, usu- ally with a little lighting bolt on it, and this key is programmed to break the link for the right amount of time for a flash. Cordless telephones flash by pressing a button, of- ten the TALK button. This flash-to-switch arrangement continues until one of the calling parties hangs up and the call reverts to a normal two-way call. If the subscriber with call waiting hangs up to end a conversation with one caller and the other caller stays on the line, then the telephone rings back with the other caller on the line. This callback feature works even if the subscriber has not flashed yet. Adding call waiting adds more states to the diagram shown in Fig. 16.3. In this case, a TALKING telephone can have an INCOMING-CALL stimulus and change to a state we will call TALK1. The telephone user is informed about the change of state from TALKING to TALK1 by the beep interrupting the voice telephone call. Unlike the TALKING state, the TALK1 state responds to a flash to talk to the other party. From the TALK1 state the FLASH stimulus switches to the other call-waiting state, TALK2. Another FLASH stimulus sends the telephone back to TALK1. Fro