So the big change between modern phones & the Lucky Goldstar phones lions used for 10 years is the goldstars detected the short circuit created by the ground ring while all the new phones detect a resistor between ground & microphone.  A 5k resistor allowed the modern phones to use a wired headphone, but the circuit has become quite complex for what it does.

Sadly, new phones no longer allow tapping call audio to draw a VU meter.  They allow drawing a VU meter for voice over IP only.