Galaxy S9 - Notifications dialog pops while using phone call keypad

[Android Central Question]

When on a phone call, while entering the numbers to reach call centers (such as 'press 1 for English', the notifications panel/alert keeps popping up so I have to click on the "phone keypad" constantly. How can I shut off that notifications panel/alert while I am on a phone call?

I do not want any notifications to show while I am talking on the phone or entering keystrokes.

Galaxy S9