Why didn't she ...

That shelf would've been too heavy to use with just one hand handcuffed to the bed. Look at just how much went into folding a piece of paper...

As for the phone, she passed out very quickly after sliding out of the cuffs. And after that I would've ran out as soon as I could too if there was a big scary monster man about to pay me a visit.

The only thing she needed was a voice. Which is why this movie should have taken place in the 90s or 80s. Seriously, Since when in today's world do you have to lift a finger to call or text someone if your SMARTPHONE is right next to you? Was the phone dead by the time she got herself free? Yes. But was it dead after she realized he was? Was there ANY hint at all that her phone was dead the entire time?

Just watched it for the first time the other night, and B kept annoying me. Maybe because King didn't write it that way? Maybe because on film it wouldn't make for as good a drama? Can't think of any other answer, but that was going through my mind the whole time.

Not sure that the shelf would have her reach the phone, but she could get it down.
I thought about the shimmy-the-bed thing too. I kept expecting her to do that. Perhaps the bed is too heavy for that; but it's worth a try. (I don't think she had enough room to move to make it work... But she does not have a ton of trouble pushing the bed later.)
The husband said that the bed-posts were "reinforced," but who reinforces a bedpost?? (And who knows that their bedpost is reinforced?) It should be regular wood and I thought she should be able to break it with her legs fairly easily (assuming it does not have an iron rod in it).
And yes-- charge the phone makes much more sense. Stupid to drive that weary. You'd feel stupid to get out of the cuffs only to drive off a cliff...