Oh yes nailed it.

That's one problem with a fixed on call rate that some organisations offer. It's a hefty chunk of cash and sounds generous to the engineers. But the cost is already known and sunk up front and not proportional to the amount of call outs so the business sees it as a fixed operational expenditure rather than an appraisal of how fucked things are.

The performance metric quickly becomes how many people you still have on cover who haven't quit to work somewhere else because they are burned out.