Addition. My suspicion was spot on. If you replace the preinstalled dialer system app with the above app and reboot the phone, then receive a phone call BEFORE you first log in, you cannot accept the phone call. It keeps ringing, you can click it away with power or volume buttons to dismiss, but you can’t accept it.
If you unlock the phone once, then lock it again, the app will work and you can receive calls normally.
By the looks this seems to be a general restriction of Android 9. After a reboot, before the first unlock, NO apps installed by the user will work, but preinstalled system apps disabled by the user will be DISABLED. This applies to vital system apps such as dialer (needed to receive calls) or the keyboard (potentially needed to unlock the phone if password protected)
This is IMHO a serious issue of Android 9, and something that needs to be addressed, either in a patch/update for Fairphone’s stock OS, or a FOSS version for FP3 for the hardcore users. ( don’t think Google would address this, I bet this is a “feature” for them as it makes it harder to replace their stuff with 3rd party apps)