From: "David VanHorn" <microbrix@gmail.com>
> You're allowed a few microamps or so.  You can also draw power when
> you're off-hook, or when the line is ringing, to charge up a supercap,
> if you can do it without upsetting the impedances.  Constant current
> sources are your friend.

To make clear what David said:

When the line is on-hook you can draw _very_little current (otherwise the 
Telco thinks you have gone off hook). This is a matter of microamps. The 
good news is the on-hook (open loop) voltage is pretty high, somewhere 
around 48V.

When the line is off-hook you can draw quite a bit more current (but of 
course the voltage will be quite a bit lower).

You can also extract power from the ringing voltage (about 20Hz AC, maybe 
100V or so). The source impedance is pretty high and the Telco only allows a 
certain load on the line (measured in "Ringer Equivalences").

---- Bob Ammerman
RAm Systems

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