[dankoss]

One way to think about this is traveling to places where connectivity is literally unavailable -- national parks, mountains, etc. Your boss cannot prevent you from traveling to those locations, so even if you aren't somewhere where you are unreachable, why should you be expected to be available?

Most of my bosses have left their contact information for when they are away from the office, but I've never had reason to use it. I would feel bad interrupting someone's time off except in cases of true emergencies, which might happen more often in other company cultures or industries.