It has to do with the effects of electrolysis. This was established in the early days of telephone usage. I’m not going into the chemical/electrical effects of electrolysis, but it is a ongoing effect which causes damages to the telephone company plant equipment and outside plant cables. The voltage is negative forty eight volts and referenced to a positive ground potential. ( -48volts and + ground) This is the voltage used to furnish a local telephone subscriber loop current to dial from their telephone using a rotary telephone and also carry the DTMF (dual tone multi frequency touch tones from a touch tone telephone. Ringing voltages used to be 105 volt, 20 cycles AC voltage. Many digital switchs now supply 90 volts, 20 cycle AC volts for the Ringing voltage used to ring a subscriber telephone set. Hope this helps some. Good luck and may the good lord take a liken’ to ya’.