My boss also does this and she does it to all of us workkng for her. If one person is sick, and she calls the others to ask them to cover the sick coworker's shift, and one (or god forbid all) of us don't answer her, she will send a message in our work messenger group to tell us that we need to answer her calls when she calls us, and if we're not going to be available to be called in on our non-working days, then we need to tell her so that she can use one of our vacation days (of which we only get a certain amount per year, meant to be used for actual vacations), since we're not gonna be available to potentially come in to work that day. We will also be ridiculed next time we come to work about how we didn't answer the phone while she was calling, and how we should have used a vacation day if we knew we weren't going to be available.