Phoning home is where a web site uses Google Analytics for example, while a CDN is where the assets you actually want are loaded from a server with large links to Internet backbone near you. Completely different things, which are sometimes conflated by people confused about the nature of the argument.

Calling the phone home feature a required library is part of the malintent from Google. Most websites I interact with don’t need a Google Analytics in order to function, for example. If I block a Google Analytics they refuse to work. The simplest explanation here is that time-short programmers just copy the code presented by Google, who in turn want everything to report back to them so they can monetise third party sites.

Building your site for AMP and blindly using the 8s timeout from the Google copy and paste archive is exactly the same story. You don’t need that much time to load all the assets, that timeout is there only to punish people who block the AMP resources.