On some phones, the incoming call screen may not light up when a call arrives. In that situation, you cannot see the incoming call dialog and you have no way to answer the call. Sometimes pressing the Power button wakes the display, but on phones with heavily modified software or aggressive battery management, even that may not help.
Why this can happen
One possible solution is to disable the phone’s lock screen.
Disable the Android lock screen
To do this, pull down the notification bar and open the Android system settings. In the system settings, find “Lock screen”. On the next screen, choose “Screen lock type” and select “None”, meaning no lock screen.
Check incoming calls again
After the lock screen is disabled, incoming calls should work normally again: the phone should wake up and show the dialog for answering the call.
Also try: allow BIG Phone to appear on the lock screen
Disabling the Android lock screen is one possible fix. On some phones, especially Xiaomi / MIUI phones, you may also need to allow BIG Phone to start in the background and show itself on the lock screen. See the separate step-by-step guide: BIG Phone black screen on incoming call fix.