in land line phones y do we hav -48 volts as operating voltage?

Answer 1

to make sure the phones are all ringing in the house!

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Answer 2

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). Batteries are needed in telephone central to make sure that it operates also when mains voltage is cut and they also give very stable output voltage which is needed for reliable operation of all the circuit in the central office. Typically the CO actually runs off of the battery chargers with the batteries in parallel getting a floating charge.

The line feeding 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.

Some countries use other voltages in typically 36V to 60V range. PBXes may use as low as 24 Volts and can possibly use positive feeding voltage instead of the negative one used in normal telephone network. Positive voltage is more commonly used in many electronics circuits, so it is easier to generate and electrolysis in telecommunications wiring is not a problem in typical environment inside office buildings.

Some older offices employ battery reversal (swap DC feed to tip and ring) to signal off-hook at the remote end.