Post History

An alternative to creating a userscript is to use a bookmarklet.  This is a bookmark that contains an executable snippet of Javascript code.  When you click on it, that code will get executed in the context of the current page.  This has a few advantages vs. a userscript.  You can easily control exactly when and how many times the script fires, and you can use it on any page (not just those you pre-configured the userscript to run on).

Leveraging the code in your answer, create a new bookmark in your browser and use the following as the URL:

    javascript: (() => {document.querySelector(".cbntphone_icon").style.display="none";})();

Place that bookmark on your browser's "bookmarks toolbar" for ease of access.  Now, whenever that popup appears, click on that bookmark and the popup will disappear.

I find this particularly helpful when modifying pages with dynamically generated content (i.e., ads that get created when reading down a page that uses "infinite scrolling").  Whenever I see a new ad get injected, I hit the button again to find and delete the ad.  A userscript would run once, and any ad created after that point wouldn't be affected.