m either call-waiting state, TALK1 or TALK2, having ONE-OF-THEM-HANGS-UP changes the telephone call back to the regular TALKING state. If the TALK1 or TALK2 tele- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Call States Call States Hang-Up TALK 1 RINGING Flash Flash Hang Up QUIET TALKING They Hang Up TALK 2 Hangs DIALING Figure 16.3 PSTN call states with call waiting included. phone has a HANG-UP stimulus, then it goes into a RINGING state with the other party on the line. This is a feature of call waiting where the subscriber who hangs up gets called back with the other party on the line. Three-way calling is a custom calling feature that allows a subscriber to talk to two other parties at once. The subscriber flashes to make a second call, the first call is put on hold, and the subscriber gets a dial tone. This dial tone usually starts with some tone pulses to make it sound distinctive, and this is called a broken dial tone. The sub- scriber can dial another telephone number and wait for it to connect while the first call remains on hold. This second call can continue for a while until the subscriber with three-way calling flashes again and all three parties are on the same call. This is dif- ferent from call waiting, where flashing alternates the two calls and the three tele- phones never share the same line. Often we say the second flash conferences in the other party from the original call. Another flash from the three-way calling subscriber disconnects the second call and leaves the original call intact. If the three-way calling subscriber hangs up, then both calls end, but if either other party hangs up, then the call continues with the two remaining parties as an ordinary two-way call. In some office systems run by a private branch exchange (PBX), when the original three-way calling subscriber hangs up, the other two parties remai