I don't know that we've talked about the specifics of the oncall compensation program publically, but the broad strokes:
You get an additional percentage of your base pay for the hours you're oncall outside of normal business hours (evenings and weekends). The percentage is based on how tight your response SLA is: if you have to be hands-on-keyboard in 5 minutes after a page, that's obviously a lot more disruptive to your life than if it's 30 minutes, so the compensation is adjusted.
You get an additional percentage of your base pay for the hours you're oncall outside of normal business hours (evenings and weekends). The percentage is based on how tight your response SLA is: if you have to be hands-on-keyboard in 5 minutes after a page, that's obviously a lot more disruptive to your life than if it's 30 minutes, so the compensation is adjusted.