We operate as per ICAO and our book states----
Flight Level and Altitude Reports by Pilots The vertical separation standards applied by ATC stipulate one aircraft may be assigned the Flight Level or Altitude previously occupied by another, after the latter has reported “Leaving”, except when turbulent conditions exist,. The word “Leaving” means that a positive movement has been observed on the altimeter. This call shall not be made until climb or descent has commenced. “Leaving” is not a statement of intent, it is a statement of action. The call is not required when under Radar Control.