Hello, Please do mail me about the following questions:-
1) Firstly is there any circuit diagram avaiable here which can be use to make a simple intercom system home intercom which works on two wires with sound musical sharp sound bell. Like for making good sound we need to use four wire , what is the theory behind this. 4 and 2 wires.
2) Is there any website from where i can get information about making a simple PABX, private automatic branch exchange.