Sideloaded a Phone App
======================
I don't want to use the default Google Phone app, so I install the
Fossify Phone app from F-Droid.
Today I hear my ringtone when the device is in sleep mode, but to
answer the phone, I have to unlock the screen, and start the app.

Yep, when you download an alternative, better configure it.
After a long press on the phone app icon, I tap the app info, and then 
select Other permissons.

Then I allow 
  > Show on Lock screen
  > Display pop-up window while run.
  > Display pop-up window

Then it's time to test the setting by locking the screen and call it
from another phone.
The next step is to swipe the white phone icon either to the red phone
icon or the green one.
Green to answer.
Red to finish.