You can flag down someone to call 911 if it is an emergency. Most people will be happy to help once you get their attention. of course if it really is an emergency seconds count and flagging down someone may cost a minute.
911 is a free call, no sim needed. So if that is your only concern a phone that you don't pay for may be fine.
Flagging someone down to call for you isn't a workable solution. For one, you could easily be nowhere near a city and need to call from the middle of nowhere. For people in the US (and I'm sure elsewhere), the cities are car centric and you could easily be nowhere near anybody out walking around.
The places so remote you cannot flag someone down are also so remote that you don't have cell service. Most roads that look untraveled still get a fair amount of traffic and so someone will be along soon.
That's assuming someone stops. A while ago I was driving down a road like you describe in the rural pnw and stopped for a person attempting to flag down traffic. They'd gotten their vehicle locked behind a DNR gate and their phone was dead. They were ridiculously grateful because they'd been trying to flag someone down for over an hour and had been ignored by dozens of vehicles, and they didn't even look sketchy or anything. Goes back to the societal assumption that everyone has a working phone: "if there's cell service, why don't they just call someone?"
This is absolutely not true for a vast part of the PNW. For example, with Verizon, I've bailed myself out of several situations where the alternative would be - wait (nobody showed up in the time we were waiting for help when we called, so hours more at least) or walk 20+ miles.
I've also been in a situation where if we did not have a sat phone in our group, someone very well may have died from heat stroke/way to many sunburns.
If you're at the point where you're flagging down a stranger to call 911 in an emergency, why not just fully "cut the cord" (no pun intended) and stop carrying a phone at all?
911 is a free call, no sim needed. So if that is your only concern a phone that you don't pay for may be fine.