It's not going to work. The amount of power you can take from an on-hook phone line is severely limited (by law and by electronics at the exchange) ans soon as you start to get into tens of milliamps (I can't remember the exact figure) the line switches to off-hook mode, and the voltage drops to about 12 volts. Also, you might want to do the calculations to work out how much money you'll be saving. If you phone charger puts out 4V at 500mA, then assumming it's 100% efficient (which it won't be of course) then that's 2 Watts. A 1KWHr unit of electricity is about what? 10p - 15p? So you can charge your phone for about 3 weeks for 15p. 5p a week. Thats charging your phone for 24 hrs a day 7 days a week for 3 weeks. For 15p. You cheapskate.
Also, I would like to point out to you that telephone companies rarely make a loss which is subsidized by the fat-cat chief executives. If you steal electricity from your telephone exchange, all the other customers are subsidizing your cheapskate behaviour.