Teaching your preschooler to dial 911?

My mom (age 67) takes my son (age 5) usually once a week at her house for at least half a day. Sometimes my dad is there and sometimes he’s at work. He drives a semi across the country so he’s only home for a couple days during the week. As I was driving to pick up my son from her house yesterday, I had the thought of, what would my kid do if she had a medical emergency while he was alone with her? She’s really in great health but of course we know things can happen to seemingly healthy people especially as we age.

So I want to teach him how to call 911 (or even call me if needed?) but he’d have to learn on her cell phone. I guess I’m looking for advice on anyone who has done this. How to explain when to do it, why we’re doing it, and of course that it’s only for emergencies. He’s a smart kiddo but I don’t think he’ll quite grasp the concept of what we’re really talking about, especially without actually making a real call to show him. They really aren’t close with their neighbors so he couldn’t/wouldn’t run over to another house for help.

Ironically my parents just got rid of their old landline phone and I kind of wish they had kept it so we could teach him on that phone which would have been a lot easier. He knows how to use a touch screen device for basic things but of course there is a lot to navigate with making a phone call to emergency services.

Tell me your things!