It’s weird how annoying “currently unavailable” means. Known unknowns are some of the worst unknowns because the voice is filled with the dark side of imagination. For estimated delivery time, my imagination when I see “currently unavailable” does not go to “oh, it will be here any minute” it goes to:
Worst is the remedy I’m suggested is “Try again later”.
When is later? Is now later? Is this later enough?
When should I call? How many times per hour is reasonable to refresh?
We should probably banish this term with real advice for humans. First, just say “Unknown” — it’s at least honest. Then, provide a plan for finding out when or what needs to happen for something to become available. Or a timeframe. Something!