Was a dept. manager for 31 years. 2 different company's. Had several rule changes over the years mainly caused by abuse by employees. In your particular case if a unplanned impacts you as far as raises, or any other negative way I would go to HR. I was in a situation like that a couple of times where when following the rules that's the way I had to handle it. Then had to go to HR for the employee to get it changed. In your case it's really hard to say on a time for calling in. The 24 hour rule I can promise you is set for the Masses ( as DP put it THE usual suspects). If I was your manager and knowing your medical situation I would take your 12 hr notice and excuse it but that's just me.