Sorry for ambiguity. What I meant is, what if the parent trusts the kid to do the right thing, and so the kid does not have to call in to the parent for an okay to hang out with a friend ?

This kind of independence might be a social plus for a kid. Something like "I don't need a phone cos I can go where I want until dinnertime". No helicopter parenting.

This only works if the kid lives somewhere with robust public transportation between their and their friends houses, or if they own their own vehicle.

Assuming neither, they're probably getting picked up and dropped off by a parent or other family member, with dropoff/pickup schedules arranged in advance.

It's less, "I don't need a phone cos I can go where I want until dinnertime," and more, "My mom will be driving 25 minutes to pick me up at 4PM, and my friends spontaneously decided to go bowling. I can only ask her when she gets here, but that would mean she needs to drive another hour and fifteen minutes."

It sounds bad for me to be anxious, when friends use each other’s phones. Why should I be anxious?

It is common for people including teenagers to use phones for personal and private things, like internet searches and private messages to other friends.

I can imagine someone who doesn't have their own phone being clumsy (or careless) and accidently dropping it and breaking the screen.