I have a morning shift tomorrow and the way I'm feeling, I'm probably not going to be able to go. If you need to call ahead two hours in advance but there's no manager there what do you do?
Do you have night crew? The grocery manager who is in charge at night answers the phone calls and will take a note of your call in.
But if you already know now that you won't make it in the morning, call now and they will leave a note at the time clock. Best is to let your department manager know, or any front end manager.
I think the 2 hr rule means that you can't call in later than 2 hrs before your shift, but you can call in sooner. The sooner they know, the better. That way they may have time to call in someone else to take your shift.