In the US, on call/overtime without pay can be mandatory for salaried employees. IT as i recall is called out specifically if you help maintain critical systems. Normally the company just has to be clear on expectations before hand. Fortunately I have had reasonable employers that don't abuse it but i do have to do on call and don't believe i have any option to avoid.
I had a salary in my last company and did on call which came with a paid component if we were called to do work off hours. We were working on that in my current company, but they just basically let me set my own hours to deal with that. Work a lot for a few days, go home early on other days. Have some downtime? Expect you might need to work extra some other time. My company doesn't abuse this, although there have been a few really critical months of crunch time in the past few years. It's basically all evened out for me.
