As IanP already said, you can't get much power from it, and your phone company might also get unhappy about it and disconnect you from the network. But that depends from one phone company to another.nikhilele said:What is the theory and methods involved in making a 5v power supply from a phone line.
The circuit from IanP will work just fine once you are off-hook. However it won't work if the line is on-hook as it will draw too much current and make the line look like it went off-hook. Typically an off-hook condition draws about 20 mA and has about 12 volts.
This may not be legal in your country. You should check with the laws. The other problem is that your current may trip the off hook condition and you will not get any incoming calls.
If you pull over 10ma you will trip the off hook sensor at the phone company.
But this circuit does not serve the purpose, it only operates when you pick-up the handset. In the on-hook position is off. And it does not convert 48V to 5V but the off-hook voltage (arround 9V-12v) to 5V.