D30,
Something of a grey area but were they calling you at 2300, when your standby is due to start at 0500, to inform you of a 1200 report then they are providing you with a minimum rest notice period for a change of duty.
But to call you at 2300 for an 0600 report, common sense, just using their knowledge if they even understand the regulations, dictates that they should delay calling you until the start of your standby at 0500 so, yes, you are within your rights to complain that they have interupted your rest period etc. but not knowing the mentality of the operator you work for might such a protest impact upon your employment status?
I once worked for an airline whereas mis-trained staff believed they could call a crew member, as in your situation, once the crew member had had 12 hours rest after their previous duty, I tried, and tried, to explain to them, to the effect, why the hell would a crew member need a (legal) rest period after a duty, to go to a bar or whatever, they need the rest period immediately before a (the next) duty!