why do telecommunication circuits use negative voltages?

Question

for example the use of -48 or -24 volts in telephone exchanges and PABXs

Answer 1

Using a negative voltage with respect to ground was to prevent corrosion that would occur if a positive voltage were used.

Answer 2

That was the traditional way of measuring voltages
when AT&T originaly came out with telco services in the early part of the century.