I have just installed the latest version of iodéOS 6.0 on my Fairphone FP4. When I tried to replace the default (phone) calling app by “Fossify Phone”, I was forbidden to do so. The (German) error message seemed to contain a web-linked text, hinting that (after clicking on it) I would learn how to permit this anyway, but the link was non-operational (a web search on this was not helpful either). I tried to fix this by deinstalling the default calling app, but that became a disaster (I became unable to make calls using my phone; I also did not find out where to download the calling app which I had just removed). Another attempt to re-install iodéOS without the default calling app led to the same disaster. Later, I had a rather similar problem when I tried to get the app usageDirect running: Same error message (with non-functional link) telling me that it would be too dangerous to allow this app to have access to the data on my phone, but that I could do so anyway if … (?) …
I have meanwhile found a workaround that allows me to get all the funtions I need, but the original question may still be of interest to others.