How do I end a voicemail call?
By what you describe, this is more likely to be related with the proximity sensor.

It is located at the top right corner of your screen, outside of the touch area.

If anything is blocking that sensor, the screen goes black. This happens to prevent the users face from touching anything on the screen during a call.

What works for me, is either using the home button or the power button to bring the screen back on.