First thought is that could move the whole thing to secure messaging, half the problem they deal with being reasonably sure whom you are talking to.

Parks/SAR shares contact info, so if later receive a message from SAR would be confident it was authentic.

Define secure messaging? I haven't see one comprehensive system that everyone can use. There are plenty of companies selling their own "secure messaging service" but no one system everyone can use.

The secure communications app Signal isn’t perfect but in my professional opinion it’s the best choice out there for regular folks. No affiliation.

What I mean though, is how do you get everyone to use the same app? If SAR in one place used signal, and SAR somewhere else uses something else.

An excellent point.

I think we’re best off educating people on what to do if they get lost.

SAR doesn't need to use just one. They can try and get it in touch using any.

Signal & WhatsApp use the phone number as the identifier. So as long as they know the hikers number. They can try get in touch over both.

You would need another SAR person just to try calling on all the different apps.

Any that assumes spammers/scammers don't figure out a why to call with those as well and the callee doesn't just ignore those too.

Once have the hikers phone number and a message to send, it can be automated, until get a response.
Spammers/scammers can't hide their phone number.

Thats what they said about caller ID when it first came out. Nothing is 100% secure, its only a matter of time before whatever system is used becomes hacked/abused.

But still doesn't fix the original issue of the hiker not answering, so he sees a number coming in on a 'secure' service and doesn't recognize it, would he have answered it?

Perhaps I'm "underthinking" this, but some of the suggestions sound apt to increase risk of introducing bumps in the system. I too am annoyed by spam calls. However, because of what I do for a living I must answer all calls even if I don't recognize the number and even if my phone notes that it is "potential spam." Some of those flagged calls were in fact appropriate and important. It takes only a moment to answer, then disconnect if the call is BS. I can't imagine any significant impact on the hiker's battery life if they did that.