i did some digging, and it turns out messages from 5 digit shortcodes often aren't supported over wifi calling. sometimes they are, but in her case they're clearly not.

This seems like a rather specific problem that isn't related to mountain people as such but services blocking "shortcodes" apparently for a variety of reasons. It is true that text and call reliability is becoming a real problem generally where you have these authentication issues. I myself in the mountains and have dealt with reliability issues.

Here's a discussion of this specific problem with T-mobile: https://www.reddit.com/r/tmobile/comments/ardcnc/aargh_final...