It's not the screen timeout, it's your hand covering the screen. Its supposed to shut off when it goes to your face so that it saves battery in a call, and you hovering your hand over it by accident can trick the sensor.
I was having the same issue and wasn't able to use the keyboard during a call. That kept me from checking voice-mail.
On another thread a site moderator mentioned the Screebl app. Checked out the lite version and Bingo I was able to clear 6 messages out of my voice-mail, back in action!
I set preferences: "In-Call Behavior" to "Screen Locked On" for my phone.
The "Screebl Disabled at Ear" option did not work for me.
[Previously replaced my screen after it shattered - slipped out of my holster :icon_ nono:].