Talking over the phones is easy but it is much harder is signaling the other end. To ring the bell you need to put 90V (RMS) 20Hz AC into the phone. Lower voltages will work (down to about 40V) but different frequencies will not. You cannot ring the phone at 60Hz.
The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line.
The circuit comprises a 3-stage resistor-capacitor coupled amplifier. When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals.
Often a need arises for connection of two telephone instruments in parallel to one line. But it creates quite a few problems in their proper performance, such as overloading and overhearing of the conversation by an undesired person. In order to eliminate all such problems and get a clear reception, a simple scheme is presented here.
The requirement for a talk switch at each end on a two-wire system complicated the design slightly. Capacitor C17 is connected in series with the remote speaker, and is short-circuited when the talk switch is pressed (R22 limits the discharge current). The electronics at the local end detects the DC path when the switch is pressed.
powered by two 9 volt batteries and uses current only when it is used. Both units are connected via a two-wire cable.
with 3 wires connection