If you want to be informed about a new incoming call during a call on the Samsung Galaxy Note 9, you should activate the "Call Waiting" function.
This function is not active by default. Therefore, in this step-by-step guide we will show you how to activate call waiting on the Samsung Galaxy Note 9:
Call Waiting - Activation on the Samsung Galaxy Note 9
1. Open the phone app on the Samsung Galaxy Note 9
2. Then tap on the three-point icon in the upper right corner and then on "Settings" in the pop-up menu
3. Now navigate to "Additional Settings"
4. Now activate the function "Call Waiting" by simply turning the slider to active.
Call Waiting is now active on the Samsung Galaxy Note 9 and you will be informed by a tone during a phone call when a new call comes in.