how to switch off screen while-on call, by default
Can you explain a little more of what you'd like to do and why?
This is a developers discussion board and therefor you should specify what do you need to know in more detail. Are you looking for a developer solution or is this a generic user question?
For end users there is a discussion board that will better suite your needs where : http://discussions.nokia.com/
Answering your question, the screen does not turn off when you answer a call because it uses the proximity sensor so it will turn off when you get it close to your face to talk. There is no way to control that from code also.
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