i recently discovered the fact that our PSTN line has a negative voltage of -44V or -48 volt.
the reason is very simple :-
The battery polarities are connected so that the line voltage is negative (-48 V). A negative line voltage reduces the corrosion from electrochemical reactions occurring on a wet telephone line.
becouse if the line voltage is positive the it would attract electrons from the ground due to its positive polarity and the metallic wire get rusting due to electrochemical reaction occurring when the line is wet.
|———————————————————–positive line above ground