You forget sleeping. Lots and lots of sleeping. I like to kid our second responders.
For all paid emergency response there tends to be contract language to the effect they can be called in for some sort of exigent circumstances like disasters. Those emergencies are few and far between. If the emergency is something that can be planned for (like a hurricane in three days) people can be ordered in. If it is an unforeseen emergency it can be negated by words like “I’m out of state”, “I was asleep with my phone off” or “I was drunk when you called.” It effectively impossible to get 100% response for a sudden emergency. If you are on call in some capacity there is generally some sort of compensation.
For extra events that are planned in advance there are always some that want the overtime. Forcing people to work isn’t always needed. However mandatory overtime does exist for a reason.