I'm having the same issue on my S8 but first me it doesnt matter if I'm using my phone with another app or if it is locked. The phone rings and rings but no option to anser displays. It sort of freezes my phone up so by the time I can get it to respond to swiping down, it's on the 3rd ring. I have followed everything in all of the solutions listed and still no luck. Also, my Do Not Disturb setting isnt on.
Our two S8 Active's on T-Mobile were auto updated on 9/16/2019 and the icon color scheme changed. We could not answer the phone. The full screen display looked the same but the botton icons did not work. No way to press the button and answer the call or reject the call. Just kept ringing.
When apps were open and the drop down menue came on the tiny answer and reject buttons worked fine.
So the phone only worked for outgong calls or when there was an app open.
T-Mobile and Samsung had no solution except a new phone or a two hour drive for an intrusive software reload at a third party vendor!
The workaround was found by reading replys here and testing. The workaround is YOU HAVE TO SWIP UP in the buttons on the full screen version. Pressing the button or swiping in any other direction does not work.
Apparently this update started months ago. WHY DID SAMSUNG NOT FIX THIS BEFORE DISTRIBUTING THIS NIGHTMARE. AT LEAST TRAIN THEIR STAFF TO PROVIDE THE WORKAROUND.
A temporary fix is available. Go to settings > general management > reset. You get four choices here. If you select "Reset settings," it resets all settings of the phone to default. This is not a factory data reset. It doesn't touch your data.