The dial of phone has revolving and touch-tone. When dialing with them, those who send out is dc pulse or double frequency signal. No matter be which kind of signal, their action is the switching equipment in control telephone exchange, let it goes finishing advocate call user and the continue between user be callinged.
If be called phone leisure, switching equipment sends a ringing electric current to it, make the phone diabolo of the other side. This is to telling each other: "Somebody telephoned! " meanwhile, advocate call an user to will hear " the sound that answer a bell " . If phone of the other side is done not have empty, switching equipment gives advocate call an user to send " toot, toot, toot... " " busy tone " .