In my current fulltime service:
We work two shifts- day shift runs 0800-1800, and the night shift works 1800-0800. If you work the day shift, you are 'on call' for the night shift, and vice versa. Leads to some issues, as the day crews work 4 days a week, and have more [56 hours total] of 'back up duty' - where we need to be within a 15 minute radius of the station. Night shifts work 3 nights, for 42 hours a week, with only 30 hours of back up duty. It's not optimal by any means, but it gets coverage completed. On call, or rather 'Back up' is mandatory...And not reimbursed. If you get called in, it's 2 hours minimum pay.
Yeah, it's pretty awesome.
Company claims there's 'No other way' to provide staffing requirements. We're kinda researching this 'back up' for 'on call'. Although our company states its withing the law, we're not all completely sold on it yet.