Using these DTOs it’s trivial to set up features like as a soft power-off button ... GPIO pin settings, especially when made configurable through a simple text file as on the Raspberry Pi ...

Since the Raspberry Pi 2 was released ... The yellow line traces the signal from the GPIO to the USB power switch. Because the USB current limit is set in software, there must be a few bits ...

You'll need a Raspberry Pi 4B at a minimum ... need jumper wires or connectors matching the Pi’s GPIO pins and possibly a small button to detect when the handset is on or off the hook.