Base compensation which you receive regardless of whether there are any incidents during the week or not, plus hourly compensation (50-100% overtime + another 40% for the inconvenience)
Also, those who have opted in are quite flexible when it comes to covering for each other for a few hours - so you can still go to the movies or host a birthday party for a kid without fear of the phone suddenly buzzing.
Result? People are doing their best to get in on the roster, as the compensation feels worth it.
Try that.