How can I make the keypad appear during a phone call? I recently noticed that, during a phone call, the dial "keypad" does not appear. This is particularly a problem when retrieving voicemail, because without the keypad, I can't give the command to play messages, delete, etc.

I am running Android 8.1.0 on a Nexus 6P device. The built-in Phone app is version 42.0.28425632. 

I access voicemail using the built-in Phone app. When dialing phone calls, the keypad appears, and I can dial the target number with no problem. During the call, there is a screen with icons labelled: Mute, Keypad, Speaker, Add call, and Hold. The Keypad icon is highlighted with a grey circle, but there is no keypad visible. If I tap the Keypad icon, the highlighting disappears, and no keypad appears. If I tap the Keypad icon again, the highlighting reappears, and still no keypad appears. 

How can I repair my settings or my phone app so that the dialing keypad appears during calls?
This is a known bug. See Google Dialer bug breaks the dialpad during calls. That’s a problem

You would need to wait for a fix or from app settings, roll back the updates to the app