This circuit was found in a 1980s US publication. It indicates when the phone is off-hook by triggering an indicator.
USB58xx and USB59xx Devices Integrate a 2:1 Multiplexer to Support Reversible Connection of USB...
This circuit was obtained from an ancient American documentation. The coil consists of 80 to 100...
In the figure we show how to implement a voltage reference with the ON Semiconductor rail-to-rail...