Waqur,
You are asking a lot here. Full duplex on a single transmission line is hard enough to achieve on a modem, not to mention a simple intercom system.
What you would have to design is a method of encoding the voice waves from the sender/receiver into a format which can be passed over the single wire simultaneously.
As you may understand, if you try achieve this in a lab without changing the format of the wave, you will get interference when you listen.
You need to setup a carrier signal at different frequencies for each unit. For example, the door unit would first encode the audio wave from the microphone and pass it to a modulator, the modulator would transmit the signal through the wire at 10kHz. On the other end you would also have a decoder to take the orignal wave out of the signal and amplify it. This unit will also have an encoder, but this would transmit at 50kHz. Hence, you now have two paths for the signal to pass through.
You will need to understand modulation, (phase, voltage, current...) and a good knowledge of FM would introduce this (I don't), as these are the methods used for transmission/reception.
Welcome to the brick wall! ;-)