When I was in high school our home phone number was published in the phone book and military recruiters called it a few times when I was getting close to finishing high school.
I’m not giving my kid a cell phone if I think them having it would endanger them. If unsolicited phone calls endanger them they shouldn’t have a cell phone. They should know what information shouldn’t be given out to strangers over the phone, on a call or via message. They should know how to block numbers and recognize calls that are best left to voicemail, &c.
Also recruiters can’t or don’t want to recruit teens on ADHD or mood meds. If you can’t be an effective fighter without medicine they don’t want to find that out in the middle of a fight.