When you move the phone away from your ear, does the screen remain blank? If so, your case might be interfering with the proximity sensor, which turns off the screen when it's held up to your ear to prevent unintentional screen touches. If your case is interfering with the proximity sensor (which should be near the front-facing camera), it might make the phone think that it's still up to your ear, and keep the screen blank.
According to the Mini's manual on page 27 (access it
here), there should be a red bar with a phone icon along the bottom of the screen when you're in a call, which is the hangup bar. Do you see it?