We're seeing reports of this after an upgrade. Sometimes it is remedied by a factory data reset, but often by merely rebooting the device or doing a live battery pull.
If your face is hanging up the phone, the proximity sensor is defective, as its job is to deactivate the screen and all things on the screen, including the end call button on the screen, when in proximity to your face.
If the area is obscured with dirt/grease or whatever, the sensor will seem too sensitive and the screen will either remain black when a call is invoked or it will go black when you attempt to use the dial keys as in a number activated menu.