Any trick or ideas for taking pictures during a conversation ?

If you are talking about having a verbal conversation over the phone then the screen should not turn off. If should turn off only when it is near your face and the screen should come back on when you take it away from your face. If this doesn't happen on your device then check the proximity sensors in the top left hand corner, they might be covered by a screen protector. If not, your device may be faulty.
 
Thanks , yep that's probably the issue it's probably covered a bit by the protector because sometimes it turns back on and sometimes not.

when I have a call I would pay more careful attention to that

all the best.