If time permits a personal touch is best, I feel. Try and catch the skipper in briefing, either in person or by 'phone or, again if time/distance permits, a personal visit to the aircraft before departure. (I never objected to a call in the hotel during the hour between call-time and pick-up). Over the R/T is a last resort?
Every airline has a different policy regarding upgrades so there is no standard answer to fit all but most airlines agree that ATC should be looked after where ever possible.