🚀 go-pugleaf

The simplest way to do it is to cut one wire of the phone line after it
enters the house before it branches to any in-house extension, and
insert a sensitive relay with a capacitor across the coil in series
with the cut line.  That way the current drawn by any phone being
off-hook will close the relay, and you can use some of the spare inside
wire conductors to run a lamp at each phone.  That's pretty much the
way old CO equipment did it.  (You need the capacitor to prevent the
impedance of the relay from attenuating voice levels.)  You could add
another relay and have a hold circuit too.

If you don't want to do it that way, you can design a little widget
running from a 9-volt battery and using a CMOS voltage comparator to sense
the voltage across the phone line.  That will typically be more than 30
volts when all phones are on-hook, and less than 20 when one or more
are off-hook.  It'll have to be a rather robust circuit, since ringing
voltage is more than 100v if you're close to the CO.  Probably about
$10 worth of parts if you bought them at retail.
	- Brian