The ability to add phone call support to your Mac, if you owned an iPhone, landed in Yosemite a few years ago. I don't recall which OS upgrade flipped the bit that starting making my Mac actually notify me by ringing every time my phone does? I swear it hasn't done it since Yosemite has it? Well after the 7th phone call today and hearing it all in perfect stereo I decided to do something about it.
So, where do you think you go to turn all that off? If you said 'System Preferences', you made a smart guess, but you would be wrong. And that's pretty much 10 minutes of my life I'm never getting back.
Instead you have to know that on a Mac what you're actually doing is handling a Facetime Audio call. Obviously. And you can't get at Facetime preferences from within System Preferences. You have to open the Facetime app itself and then go do Preferences => 'Calls from iPhone' and uncheck that.
So I guess file this under things you should already know unless you don't know them?