When you’re on vacation or taking a day off, really take the time off. That means being unavailable to those who may demand your attention at work. You don’t need to feel guilty or have an excuse as to why you couldn’t answer the phone, the reason is, you’re off.
Many time people will push through and proclaim the urgency of their request. The fact is, they probably knew you were going to be on vacation and neglected to plan ahead. A lack of planning on their part should not create an emergency for you.