I have had my HTC One Max less than a month, and it has had to be replaced twice, once because the 3G radio didn't work, and the second time because of this issue. Well, the current replacement has the same issue too. I used Android Sensor Box to test the functioning of the proximity sensor, which should be what controls this function. It appears to work, but my experience tells me that it stops working a few minutes into a call.
Your ideas and suggestions are welcome. I love the phone, but this is a problem I can't live with.
Is anyone else experiencing this issue?