When I was in flight school, some instructor told me that it is the unknown ICAO standard that you should reply to an ATC call with your callsign first, like this:
"XYZ123, climb FL350" - "XYZ123, climb FL350"
But I only know one airline that operates like this. In all other parts of the world, I hear:
"XYZ123, climb FL350" - "Climb FL350, XYZ123"
So can someone tell me the definite ICAO standard on that and where it is written down?