Are you saying it is off during calls and you don't want it to be or that it's not off and you do want it to be?
This is usually an automatic thing, based on the proximity sensor. An app called Elixir can help to determine if the sensor is working properly. If the screen is off all the time, during calls, you may have something blocking the proximity sensor - often a poorly-cut screen protector or a case can trick the proximity sensor into thinking there's something blocking it.
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