You need to got to setting ->device -> sounds and notifications and make sure "sound mode" select "sound"
then go to setting ->personal->Accessibility->Answering and ending calls to turn on "Using voice command".
You now can answer your phone by say "Answer" or "Reject".
If you want it to auto answer when connected to Bluetooth, Click on the phone icon, then click More in upper right hand then Settings. There is an option for Automatic Answering.
It's an app in the Galaxy Apps as Car Mode. It was driving me crazy searching for it until I watched an old S6 unboxing and the guy showed it under the galaxy apps.