Device is Xperia XZ1 compact, OS is Sony branded OS9. 47.2.A.11.228.
Happens every time, I also restarted the phone.
If the phone is locked, and the app is closed, it will ring, and the screen will stay dark, if I unlock the phone, even if I open the app there is no dialog to answer the call. Ringing continues until timeout.
If the phone is locked, and the app is still in memory, the display will be lit an I can answer the call.
If the phone is unlocked, and the app is closed, the "white" answer dialog comes up, and I can answer
Does that help?
Should I try to send you a logfile?