HI every body,
I’m replying very late
Sammour has rightluy pointed about chemical effect.
Actually in olden days there was over head bare cables used for telephone systems & in order to reduce the chemical effect from halogens (which are the most one responsible for corrosion)which are -ve charged and are repelled by -ve voltage. Hence -V. The figure of 48 was taken becuse it was a compromise between safety (of electrocution) & power loss due to R of cables HENCE -48V