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When you in a call and you put the headset next to your head their is a sensor that will black out the screen. If you try and make a call put your finger over near the LED notification area it should automatically black the screen out. if not then their is a issue with the sensor.
In addition to Teaboy1748's suggestion, do a search in BlackBerry World for an app called TalkLock.
- If my response has helped you, please click "Options" beside my post and mark it as solved. Clicking the "thumbs up" icon near the bottom of my response would also be appreciated. My Z10 was furnished by BlackBerry but all opinions are my own.