Here are the rules, as simply as I can state them:

1. If actual work is performed, you've almost always gotta pay. 99.99999 percent* of the time. This is so, whether the work is performed at the office or factory, at a customer's or client's site, at the employee's home, in the employee's car, in a hotel room, at McDonald's, in a parking lot, in a dark alley, ANYWHERE. And, yes, "work" can include emails, text messages, and calls from cell phones.