It's especially difficult for me because not only do I constantly forget to turn my mobile on, I forget to recharge it, I never know what the number is, and usually forget to have it with me. Plus it can't text. (That's not just me being incompetent, several teenagers have tried before tossing the phone to one side in disgust and saying 'you need to get a new phone'.) So it's entirely plausible to me in real life not to be able to call someone, or be called by someone, but in fiction my characters have to be much more together than I personally am.
The trouble is, if you don't let your characters take the obvious route, they end up looking like complete dimwits. I've twice worked with a story where a single phone call would have sorted out many difficulties. First, Anna in Nice Girls Do was stuck underground. That was easy - she couldn't get a signal, and then, as part of the escape, she leaves the phone behind. Then came Natalie in A Single to Rome. That was harder to orchestrate. The whole plot fell to pieces at the end if people could phone her up rather than meeting face to face. I had to set up the missing phone early on - and have her comment several times that she really ought to get round to doing something about the phone - so its absence didn't look too much like a plot device when it was needed.
I've done this post about mobiles, but it could be anything that an ordinary person would use which is inexplicably missing when needed. If it would be used, then use it - or set up a very good reason for its absence.