Yea, this is a teacher thing. At my last school, we were asked if we wanted to be on the chain. Last year, when I did the long term sub job, I was not on there, obviously, since I started in Feb. There was one delayed opening a couple of weeks later; I heard it on TV a lot earlier than I would have received the call.
The school I worked in before that routinely started the chain at 5 AM. At the last school, it was sometimes 6 before I got called... after I had gone back to sleep because I had already seen it on the news.