How can I lock screen when answering phone calls?

[hkkc]

When I answer phone calls on my S8+, sometimes my ear touch the screen and turns on an app, flashlight, disconnect Wifi for example.. all accidentally.

Why did android not prevent this by locking the screen when the phone is placed by the ear to take phone calls? Is there a feature that I did not turn on?

 

[hallux]

Welcome to AC.

Android does prevent this, by use of proximity sensors. What app are you using to take phone calls? Apps other than the default phone or dialer app may not interface with that sensor and the function of the screen turning off may not work as expected. It's also possible that your proximity sensor has failed or stopped working. I have an app called Elixir that gives me all kinds of device information, including the status of the proximity sensor.