First Part: Why -(negative)
Answer: To cancel the effect of noise, prevent ionic reactions etc etc.The line voltage was selected to be negative to make the electrochemical reactions on the wet telephone wiring to be less harmful. When the wires are at negative potential compared to the ground the metal ions go form the ground to the wire instead of the situation where positive voltage would cause metal from the wire to leave which causes quick corrosion.
Second Part:Why 48V?
Answer:The -48V voltage was selected because it was enough to get through kilometers of thin telephone wire and still low enough to be safe (electrical safety regulations in many countries consider DC voltages lower than 50V to be safe low voltage circuits). 48V voltage is also easy to generate from normal lead acid batteries (4 x 12V car battery in series).
Also, Telecommunications started with wireline first. Since GSM/CDMA came after it , they were designed to be compatible with the existing power arrangements available with the PSTN operators.