How to force all incoming calls to get dumped to vm *** if and only if I am on the phone ***

When I am on the phone talking with someone, I do not want my phone to tell me that I have incoming calls . I do not want anything on the notification screen, I do not want to be forced to look at my phone and manually ditch that incoming call, and I definitely do NOT want the phone to go "BOOP" in my ear every few seconds while I am trying to have a conversation. What I want is, when I am on the phone making a call, ALL INCOMING CALLS JUST GO STRAIGHT TO VOICEMAIL.

Now, to be super clear, I do NOT want calls to go to voicemail when I am NOT on the phone. I want to set one setting ONCE that tells the phone "hey, if AND ONLY IF I am on the phone, then dump all incoming calls to VM."