Some you you may have identified a short coming in the phone presented in the last post. If the line is reversed you cannot signal any of the other phones on the line, or in other words it's "polarity sensitive".
This easily fixed with the addition of a 'bridge rectifier" (also know as a polarity guard) to the electronic buzzer in each phone. It's purpose is to steer the calling signal to the buzzer regardless of how it's applied.
If your only using a few volts to excite the transmitter this will cause a new problem. A standard bridge rectifier will actually reduce the applied voltage by 1.4 volts allowing only 1.6 volts activate the buzzer. With most of the devices on the market this will be far to low for reliable operation. The solution is to add a few battery's to increase the voltage. This in turn creates another issue with the transmitter and receiver receiving two much voltage, possibly causing the carbon granules in the transmitter to "burn", or possibly burning out the coil in the receiver, not a good situation . What to do? See the next schematic. Note: I wanted to show how a phone works in general hence the previous schematics. Also some may want to make 'compact" units that are easy to carry around.
This schematic depicts a "standard" phone connected to an external battery (12V) through an inductor the purpose of which is to 1) limit the current to the phone ( 20-40 ma) and 2) to block the modulated DC from being shorted out in the battey ( yes it will). Again an isolating capacitor couples the phone to the line. This circuit is much simpler to apply to standard off the shelf phones. As with the previous design, signaling may be via a DC voltage, this time however the higher battery voltage helps negate the effects of the bridge rectifiers and long lines. The phones internal circuitry will automatically compensated for voltage variations and insure proper operation.
In the next post I will try to cover more advanced "signaling" schemes