How do I end a voicemail call? Apologies if this is an elementary question.

I have called up the voicemail service and I listen to the first message; I'm then invited to press a key to save, delete, and so forth, but the screen has gone blank and there is no key to press. How do I recover the keyboard so that I can select an option?

A second problem then presented itself; that not only is there no key to select an option, there is no key to end the call either. The only way I could do this was to take the battery out of the back.

This can't be what the designer intended, but there is no guidance given any of the manuals I have been able to find.

Can anyone give me a pointer to an idiot's guide?

Edit: It is a Samsung Galaxy (can't say what model number), and it has no protective devices attached to it.
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.