Okay, I was believing you were referring to the Mute for the ringer. Do you have a case on the iPhone? Do you have a screen protector on it? It is possible that something may be blocking the proximity sensor when you are bringing the iPhone up to your face, so that the screen is not turning off. If you have a chance, make a call to someone who doesn't mind if you test a few things. I usually use my son for that, and after the call connects, bring the iPhone away from your ear. The screen should light up. Take your hand and bring it over the earpiece of the phone and the screen should go dark, remove it and it should light up again. If not, make a Genius Bar appointment at the nearest Apple Store to have the device checked.
Now if you do have a case or screen protector, make sure that it is not interfering with the proximity sensor. For the iPhone 11 Pro, there should be a notch in the screen protector, and a case should not obstruct any part of the earpiece, front facing camera, and sensor area.