I have the 10.1 samsung android tablet with phone built in and there is no option to stop the display turning off in the middle of a call.
The main concern during a mobile phone call is - will the line drop - Having a display that turns off all confirmation that the call is still live IS a suboptimal usability issue, as the suddenly blackened display implies the call has dropped. Grappling for the power button to turn on the display is very cumbersome and counter intuitive and interrupts the flow of a conversation.
Android Developers - please make the display stay on during a phone call as default, and at minimum make it so the user can choose this behaviour on 10.1 tablet as it is certainly preferable for most people. Saving 30 seconds of display time compared to the downside of interrupting a voice call makes this an easy choice. Otherwise Android is good.
Thanks,
Paul