There is a telephony receiver in the Device plugin that sends a message to all plugins so that they can handle incoming calls etc.
Currently the only core plugin that listens for those messages is the Media plugin so that it can pause playback when a call comes in.
There is no documentation for this (before the great plugin break out, there didn't need to be) so if you just add the Media plugin, audio playback does not pause on an incoming or outgoing phone call.
I am not advocating moving this into Media as there might be third party plugins listening for this. However, I do think that at the least this needs to be documented in the docs for the Media plugin/api and possibly even have Device as a dependancy for Media.
There's another bug that is related involving android permissions but I will lodge that separately. (See: CB-5505)