What an entitled pair they are OP!
I personally HATE mobile phones. We moved house just over a year ago, and at that time decided to give up on a landline and only use our mobiles, but even now, I still don't always think to take mine with me if I go out with DH. I don't even remember to charge it as often as it needs, as it simply DOESN'T rule my world. A while back DH came in unexpectedly at lunchtime, he looked absolutely distraught, and demanded 'why didn't you answer your phone?' I'd gone out in the garden to hang some washing on the line, it was a lovely day, so I'd got sidetracked into doing some gardening, and then sat by the pond watching the fish and listening to the birds, with not a thought about my phone, and of course, he'd been expecting me to pick up, so got extremely worried. However, while I did feel sorry for worrying him, does it really mean that I have to have the darned thing attached to me for the rest of my life, just for other's convenience? I think we've all become so used to having everything immediately, that we really don't cope well when we can't have it. It doesn't matter whether it's shopping, buying a house, getting a doctor's appointment, or calling family / friends, thanks to things like Google, we've become so used to getting information immediately, that we expect everything to be like that, and then get angry when it isn't possible. In my opinion, it's become absolutely ridiculous, after all, do I really need to take my phone with me to the toilet, on the off chance that someone will call me, and get upset because I don't answer immediately?
For the most part, my family have now got used to me forgetting to take my phone with me, and don't tend to worry like they did when I first had it, but even so, my DH came home from the shops the other day, and demanded to know why I hadn't answered my phone - I'd been putting the vacuum cleaner round, and hadn't heard it.
I'd be very interested to know whether the OP's MIL always has her phone glued to her side, I bet she doesn't.