I think a landline is 5 or 6 volts, which is enough to power LEDs and even recharge a cell phone, if you rig it correctly (hey, that may be the definition of irony right there).
Question: If I do not subscribe to a landline service, will the phone company know I hacked into their box and am stealing their 5 volts?
If you have no dial tone then you have no voltage. Can't hack something that isn't there. Unless you are talking about going to a main box.
Ring voltage on a copper line is over 100 volts. It WILL shock you.