(i) Positive voltages cause comparatively more corrosion in metal than negative voltages.
(ii) Negative voltages are safer for human body while doing Telecom activities.
(iii) Lightning may cause positive voltages in the equipment circuitry. In that case, negative voltages (lack of electrons) neutralize positive charges and prevent excessive heat.
(iv) Negative voltage is safer for long telephone line for transmitting power trough it.
It is -48 V D.C cause of the commercial battery stack available. Commercial battery stack is available in -12 V, -24 V or – 48 V. For info, actually any telecom equipment or for that matter any sophosticated equipment runs on a range of supply voltages by using their own inbuilt SMPS and it is very difficult to maintain a constant voltage by the power source.
Can negative voltages be used in other domains like power systems preferably to protect equipments from lightning and noise as has been identified in above comments..