How can I detect whether the current user is on a call?

Question

I'd like to be able to detect if the user is on a voice/video call, whether it's Skype, Slack, Zoom, Highfive, etc. I'd like to be able to tap into what Windows itself uses for reducing volume during a call. I've tried searching a dozen different ways, but can't find any info on this. There must be a standard event that these various apps are using, but I can't find it (ignoring mobile/phone-specific). It seems like the answer might lie with TAPI, but how likely is it that these services would all plug into that?