In normal state, telephone no.1 and telephone no.2 can be used as an intercom when telephone no.3 is connected with the phone cable. To be used as an intercom, pick up telephone no.1 and push switch S1, then piezo buzzer 2 will sound, and so on.
Components :
Resistor R1 : 4.7k ohm
Polar capacitor C1 : 2.2uF/100V
PZ1-PZ3 : Piezo buzzer
S1-S2 : Push button
S3 : ON/OFF stereo switch
12V DC/200 mA power supply
