Proximity Sensor iPhone 12 Pro

Do you have a case on your iPhone? If yes, remove it and do some test calls. If the problem goes away your case is interfering with the proximity sensor. If it stays then click on the Get Support link at the top right of this page to start the service process as you have a hardware problem with your proximity sensor.

Are you using a different case than you used on the other phones? If you take off the case does it work faster?

I tend to unknowingly activate the mute button with my cheek right at the beginning of a call due to this.

I only notice when I realize the other person cannot hear me, or hangs up. Really bad for business calls.

I have deactivated the Attention Aware feature, as someone suggested, but that didn't change anything.

I now try to start or answer calls with the phone a few centimeters from my ear, so as not to touch the screen with my cheek while it is still on. Cumbersome, really.

I've never timed it and it has been awhile since I used a phone with a home button.