Is someone covering the full 24 hours, or is it just out of hours?

Are they under a one week in X rota, or expected to be permanently available?

Are they expected to cover anything/everything, or are they just the escalation point for their specialist area?

What other support is available? (i.e. if the shit really hits the fan, are you left to deal with it alone?)

On average, how many times are callouts expected? There's a big difference between half a dozen times a week and half a dozen times a year.

How are the extra duties remunerated/compensated? Is there time off in lieu?

There's a massive spectrum there ranging from hugely unpleasant and not worth the money to not particularly onerous and helpful extra cash/time off.