Are there certain fields where people are implicitly required to be "on call"? As a software dev (in an oncall rotation) even when I'm not on call I still am ex

Are there certain fields where people are implicitly required to be "on call"? As a software dev (in an oncall rotation) even when I'm not on call I still am expected to check in on things and be available during non-business hours. Same with the IT folk in my company. However, does the CFO have this same requirement? Couldn't there be some financial-related mishap in the middle of the night? HR? What if an employee, say, assaults some other employee off-site in the middle of the night? Sales guys? ie, one of their customers is irate off-hours. The president? I've never bothered to ask.