When I was Oncall it was 1 week in 4, I got paid £56 a week extra oncall + whatever extra hours I worked. Some of my Colleagues were on different contracts so got better oncall (20%) salary hike that week.
We were expected to be an hour max from our vans (so didn’t matter what time of day the phone rang I was ALWAYS riding, out or walking the dog).
Min callout 2 hours if it was something we could fix over the phone/or it was rubbish, or 4 hours if we had to go anywhere. Then once we’d hit our 12 hour limit for the day, so if you’d done 7hr standard day then max OT was 5 hrs, it was ring manager to request extension and that was usually a NO or 14 hrs MAX, then 12 hrs rest before the next call.
Ask your Boss if a fatigue based accident would go down well with the HSE.
oh and if I really didn’t want to be disturbed the phone would get wrapped in foil and put in the fridge. That way if it’s a company phone and they look at the logs they won’t see it being switched Off, just a return to coverage message when you unwrap it.