Yes I totally agree, it's all very scary.
I rely on my Dad answering my calls on the landline now, if I don't call them, they don't call me. It's frustrating and a worry, especially as I live 300 miles away.
I totally totally appreciate your concern. I am 250 miles away from my father and the same as you, we can't visit as often as we should. Dad lives alone, has a landline, a very basic corded phone, which we call him on. It's in the dining room so if he's in the lounge with all doors shut, he doesn't hear it. (I've suggested keeping the doors open but that falls on deaf ears and there's no phone socket in the lounge.) He did have walkabout phones but every time there was a storm, it would blow out the base unit so to avoid that we just got him the corded phone otherwise I would tell him to take the phone into the lounge with him - but of course, he wouldn't remember to do that anyway.
He does have a mobile but it's never charged. We have told him to keep it plugged in but again, falls on deaf ears. He refuses to have the internet, has never had any interest whatsoever in technology and really, he abhors it so no chance of having Alexa or anything internet connected.
So.. so far Dad is still fine using the phone. I call him once a day, he calls me perhaps once a week or less (when he remembers). I try to call him when I think he's going to be in the kitchen (dining room) either finishing dinner, or getting ready for bed so that's usually about 9pm. In the past, if I've not been able to reach him for a few days then I've had to call his landlord or neighbour to see if he's ok! He of course totally oblivious about my anxiety.
I eventually went to see his neighbour and kindly asked for their contact details, and shared mine, and asked them to call me if there was anything unusual, i.e. curtains not open by a certain time (bedroom and lounge), or car gone for a long time. It's reassuring to know what they will keep an eye out and alert me if needed. It's a 5-6 hours drive down to Dads'.. probably same as you. Not easy, is it.