Let's offer reasonable overtime
Here's a suggestion, when ASA call to interrupt your valuable time off, and you're tired/just had a beer/engaged in an otherwise unimportant task-say out paying bills to ensure continued supply of power..phone..etc,
say, "ok, I've just had that beer/I'm tired, but I'm prepared to come in and work, BUT because of the above, I'm afraid I'm only available to do non-operational duties."
In this case, you haven't refused reasonable overtime, but you have then complied with your legal obligation to work only when fit and able.
This being the case, there would then be a slight possibility that you may be unable to return to work operationally on your normally rostered shift due to being fatigued unnecessarily by working on your day(s) off.
Worth a thought,
Cheers WMF