How Can I BLOCK RingCentral/Zoom/Skype etc from changing Default For Calls??

Hi there... this is driving me crazy. My clients use RingCentral for three different meetings I have during the week, every week. Every time I run RC on my Mac, it changes my "Default For Calls" setting in Facetime to use RingCentral instead of Facetime.

I don't want that, and I've never authorized it. I only want to use RingCentral when I have to. Otherwise, when I make calls with my Mac via the iPhone, I want to be able to dial using the native setup.

I see many links explaining how to change it back once it's happened. But I know how to do that (inside Facetime/Settings).

What I want to know -- and I'll bet others do too -- is there a way to permanently block RC and similar third-party apps with messing with the Default For Calls setting? I'll go as far as a terminal command if I have to.

Currently, what I've been forced to do is delete the RC app, take all meeting links, and paste them into Chrome, as RC's web version doesn't play nice with Safari either.

Help?