I don't know. I have call guard, but don't use it. I assume you have touched the screen on purpose to make sure call guard is working properly? You can just tap it with your finger.
But as usual, I have to always ask what is actually happening when you have trouble. If it a call being cut off, it may not be the sensor. If it is activating speaker, mute, or something like that, then it probably is.
Another thing you can do, though it is a pain, is video yourself on the phone if you are in front of a system with a camera (I have one in my Apple monitor). I turn it on to record while I'm on the phone and, if something happens, I stop it and can go back and watch that part of the video and see if it came on.
But you'll probably need to have the phone's brightness turned all the way up and, especially if in a bright room, try to keep an angle to the camera that lets it more easily see what happens. That makes it a pain. If the room isn't bright, even if the camera can't see the screen, if the phone comes on, it might be bright enough that you can see it's light showing up on your face. Of course, you can't be one of those people who literally have the screen pressed against their face!
