This is cool! Though there's some irony in using a whole separate door sensor for the "off the hook" detection, when the phone already has such a sensor in it.

This is cool! Though there's some irony in using a whole separate door sensor for the "off the hook" detection, when the phone already has such a sensor in it... IIRC, when the phone is on the hook, it appears as an open circuit, and when it's off the hook, it's a closed circuit (current flows)? Not sure of the details, though.