How sigstop knows you are on a call, with zero permissions

A break reminder has one job it must never get wrong: it must not fire in the middle of a call. So when I wrote sigstop, a menu bar break reminder for macOS, the first question was how an app can know a call is happening without the Microphone permission, which would not help anyway: that permission is for capturing audio, not for knowing whether somebody else is. This is what the code does, with the caveats the docs state next to it. CoreAudio exposes a property on every audio device called kAudioDevicePropertyDeviceIsRunningSomewhere. It answers one question: is some process on this machine