internal phone (help)
your requirement is not clear . What you would like to do ?
To get all phones listen to each other just out electronic gyrator or simple 1 - 2 kOm 2 watt resistor to DC 48 volts and connect phones in parallel with one sode connected to resistor and another to ground . That is simplest and it works . But if you want to improive speech quality (the volume will be decreased ) - you should supply every phone through separate resistor and connect them via capacitor. To get better performance you should preamplify each telefone signal and then sum them.
Novadays conference call devives are done in form of dsp where sum and echo cancelling is done in software .