I don't know if you still need help with this, but I had the same problem until about 2 minutes ago.
What I did, on my Accessibility settings, I had the "Power Buttons ends call" feature checked. I unchecked it and then tried calling my voicemail. Two seconds after I hit talk, the screen blacked out. I pressed the power key and voila! The dial pad came on. Problem solved.
Hope I (or someone else) helped!