So why would somebody *choose* to leave their cellphones at home? I mean, even leaving out the fact that there's a serial killer on the loose who has already proven to have an unhealthy interest in you, why would a couple - especially parents of a young child - willingly put themselves in a position where they couldn't contact anybody even in an emergency?
I get that they probably didn't want anybody to be able to call them, but that's what caller ID is for. Somebody from work rings, you just leave the phone on silent and wait until they go away (not that I've ever done that when on a day off...;-)
It just seems like a case of characters being hit with the stupid stick in order to make the plot work - and if you have to write the characters out of character to fit the plot, then maybe the plot was a crappy one in the first place.
Hell, why not just have them be in an area with crappy coverage.