How much power is in my phone line?
When on-hook, you really cannot get much power from the phone line. A very few
microamps (load on the line should be something like 5Meg or greater.) There
was a time (like on the princess phones) when the phone company actually
supplied a power wire-pair for the phone lights. But that didn't last so long.
If there was any appreciable power availability from the phone line, can you
imagine how much power they might have to fund without anyone to charge for it?
When off-hook, they are supplying power. So you can draw some current when you
are engaged in a phone call. But the total loading (including the phone you are
using) shouldn't go under some hundreds of ohms at a subscriber site, if I
recall.
Is there any way I can use it?
That's a wide-open question. You should have clarified it by providing some of
the specific ideas you are thinking about. How can anyone well answer such a
question as you ask here?
The general answer is that you shouldn't consider it. Dbowey is an expert on
this subject, sometimes posts here, and was a real help in letting me know just
how difficult even this task really is, if it is to be done well and correctly.
I wanted an LED indicator that wasn't part of any phone and that I could put on
every phone jack in the house to provide a positive indication if the line was
in use. My resulting circuit draws less than 1uA of current when the phone is
on-hook and draws just 1mA for the LED when the phone line is off-hook. Even
then, I can't add too many of these around the home as the LED draw itself could
wind up "holding" the line after the phone is replaced to the on-hook condition.
I considered the idea of automatically interrupting the LED load every so often
so that I could test the line, allowing a larger number of these to be used
simultaneously, but I'd need to include time for the central office to respond
and I'd need to work out some way for all the LED units to remove their loads in
concert with each other, too. Way too much pain.
The phone system has been carefully crafted over time exactly for one purpose
and probably also to minimize their operational costs due to problems or abuse
at the subscriber end of things. They've got it pretty well cornered, now.
Jon