Re: hmm
"My parents are in an extremely similar situation. They live in the middle of nowhere..."
I am in the same situation. I'm also in the middle of nowhere, pretty much. I think it's over a mile (!) from here to the Cambridge Science Park. Depending on weather conditions, there's no mobile signal anywhere in the house on any provider. We have to walk out into the street to send or receive SMS messages.
Rather than wall reinforcement, we've gone for a wired connection and WiFi calling, though the latter is flaky enough that we should probably consider the former as well.