I call one hour before start time if they didn't already text me. Sometimes they say to call back in 15 min. Fine, no problem. However, once in a while they say I'm not needed, then call me at 11:00 or 2:00. I don't answer, and they freak. I got into a big argument with the center manager about it. I'm not on call all day unless I'm getting paid for it. I couldn't find anything in my supplement about it, but that's how we do it here. I would think you're only required to call once. If they ask you to call back or they call you, that's up to you.
One piece of advice. Don't let them make you think your on call all day. You're not. 15 minutes becomes 30, then becomes an hour, etc.