These extensions use MellowTel-js. After this article from ArsTechnica went live, the developer responded in full detail and transparency.
If you’re a Dark Reader user (as that’s one of the most widely used extensions), definitely read MellowTel’s response on how their technology works. It made me realize the Ars article was not fully vetted.
https://www.mellowtel.com/blog/responding-to-ars-technica-and-mellow-drama-article
Edit: Dark Reader on this list is actually a knock off version just for Edge browser only - it’s not the widely used Dark Reader that’s on multiple browser engines. See another user’s comment that replied to me.