I doubt it's illegal in many places.

What is common is that being available counts as work.

Your work hours are specified in your work contract. Demanding service without compensation and contract sounds highly illegal.

We have on-call duty for this. It should perhaps be compensated differently, and certainly one person should not always be on-call (though I have been at times on-call for months at a time, but for a thing that normally does not page at night). But what are we do to, have staff around the world so that someone can be on-call during their local business hours so we can have 24/7 support?