All very well proving how brave you are by answering the phone preparing to say "NO" .... But remember that it could well be a future O Day they are after or bringing that Reserve or Reserve call out forward. If you are given 12 Hours notice you have just monumentally shot yourself in the foot.
It is also not unheard of for that emphatic "NO-SORRY" to a G Day request to find its way onto your P.File with a special note from the Crew Control Supervisor questioning your unwillingness to help.
Much easier just to let the phone ring out and pick it up the minute your reserve starts ... or if you really want to go to work on a G Day pick up a Proffer. If it is anything longer than a TPE you will be much better off than a G Day call out... and you will help out a colleague (strange concept hey!)