What trouble? Adults don't want to mess with kids (most child abuse happens with close aquaintances, not random crazies on the street), so their biggest threat is going to be bullies around their age, and a parent stepping in will just make that problem worse, so they'll need to learn to deal with that on their own anyway.

I personally have never had an issue going to/from school, other than the typical bullying on the bus. The most likely problem they'd run into is getting hit by a car, in which case they need paramedics and police, not me, and those emergency services will call me once they identify them (and I trust random strangers to call emergency services if a kid gets hit).

The only time I expect my kid to need to call is if they're at a party or something outside of school and need to be picked up because they don't feel comfortable. When they get to an age where that's a thing, we'll have a loaner phone for them to use.