When you're in a committed relationship, do you think there's an understanding that you're ALWAYS available by phone?

I was thinking, what if you just want to shut off that cell phone and be unreachable for an evening? Not forever, maybe just one night, and maybe just to watch some crap TV or take a nap. In today’s society of fewer and fewer land lines, is there some sort of expectation that you should always be available by cell when your honey calls?