If you want to be informed about a new incoming call during a phone call, you need the "Call waiting" function.
If this option is activated, you will be informed by a tone that a new call is coming in.
This is very useful because you will never miss an important call, even if you are on the phone. Where you can activate call waiting on the Samsung Galaxy S8, we want to describe this in our article:
Instructions to activate call waiting on the Samsung Galaxy S8
1. Open the phone app on the Samsung Galaxy S8
2. Tap on the three-point icon in the upper right corner and then on "Settings"
3. Now navigate to "Additional settings"
4. Now activate the function "Call waiting" by setting the slider to active.
Now the call waiting is active and you will be informed about new incoming calls if you are currently in a phone call