Down Under, we have in our enterprise agreement a provision for "Grey Days". Basically you are rostered for 8 hours, but available to provide up to 8 hours cover during the 16 hours from the start time of the grey shift. 8 hours are paid/acquitted in the roster - whether called in or not. They were not used a lot previously, but have been quite popular since COVID reduced both roster requirements and the overtime budget.
We also have a "voluntary on call" provision in our agreement that has never been used (to my knowledge). Basically an individual guarantees to be available (in 24 hr period) for call in for additional duty in return for a few (I think 4 hrs) pay. If called in, they get the normal additional duty rate for the hours worked.