I used to watch that show but never noticed. I don't tend to notice things. lol Since their kids were young enough to be in the house overnight, I'm going to make up that maybe the husband had sick, elderly, parents who were likely to have a medical emergency. I'll also make up that maybe her parents were either dead or young enough to be reasonably healthy. lol
All I know is that when my three kids were little I was the only one walking the halls at night when there was reason to watch one or more of them; and I was also the one up doing my own work (which I did while I wasn't working full-time outside the home, but which required use of my phone only occasionally and with some projects.
Even today, though, with three grown kids (thirties), I'm still the one (between my ex-husband and me) who sleeps with, and keeps, two phones in order to make sure my kids can get in touch with me at all times (even if that's only for something like, "Hey, I'll be in the area and thought I could come by for awhile.". Having had those times when one was in the hospital is, of course, another reason I want to be able to be reached at all times.
More input than you were looking for, I imagine; but now as I think more about that show, I recall that I didn't really like that husband of hers anyway. In real life, those husbands who make sure they grab the phone when it rings are actually often control-freak, abusive, or at least self-important husbands who "screen" calls from "the outside world". (which often means from the relatives and friends of the wife).
Then again - and in fairness to the show - maybe the idea was that since she up all night much of the time, talking on the phone, he'd try to get calls that "didn't apply" to her so she could get some sleep. (I tend to doubt that one, but I'm nothing, if not fair.)
Oh well... With about seven feet of snow outside in my area, and more coming in the next few days; I just thought I'd have some fun with this question.