The connection between any two subscribers is through a Local Switching Exchange (or) Central Office.
It is a hard wire connection in series with the earphone and carbon microphone of each telephone handset with a battery. The Local Switching Exchange or the Central Office (CO) employs a battery plant.
To make a phone call, a subscriber lifts his handset (receiver), so that a switch called “off-hook” switch in his telephone set is “on”, and he gets connection with CO. The CO sends a dial tone signal to his handset. As he dials a number, each digit is identified as interrupted line current at the CO. The CO with the help of a panel full of digital circuitry examines whether the line corresponding to the digits dialed by the calling party is free. The line will be found free only when the off-hook switch at the called party’s hand set is off. If the line is free, the CO – immediately places a “ringer generator voltage” (50V 20Hz, “On” 2 seconds, “Off” 4 seconds) on the called party’s line as well as part in the calling parties’ line. This timing is different for outer exchange calls. We hear only this ringer sound, being sent from the CO. The timing is different for outer exchange calls.
When the called party at the other end lifts his handset to answer, the off-hook switch in his handset is “on” and only now, his (handset),receiver gets life and making him capable of hearer speak. So the receiver the other end is enabled only when it is lifted there by making the off-hook switch “on”.
As long as the conversation between two subscribers being established, it is the CO which communicates with the calling party just as sending.
