I think an important question to ask is how they got his number in the first place; it's possible that they could find the new one in the same way

You're talking cell phones, which is perhaps a reasonable assumption in this case. Historically, phone numbers were public in white pages by default though.

Fair enough! I generally think of cell phones when I see the word "voicemail" because we never used that term to describe the messages left on the answering machine my family had when I was a kid; they were just "messages", which at the time was unambiguous when talking about phones :)

That can trigger the stalker to escalate.

It could also result in the stalker never calling again. I guess it’s important to understand the probability of outcomes before just listing negatives.

Personally if it’s something potentially harmful, I’d rather have it where I can see it.

How? They wouldn't know unless he'd drop the old one. I imagine he isn't calling the kid back to say "hey, got your voice mail, sounds good, keep it up", so the kid wouldn't know that their voice mail goes straight to /dev/null while he uses a different number.

Don't feed the trolls. Being fearful of blocking unwanted calls, so you allow the calls to continue just feeds them. You are telling them this is acceptable.

The trolls have already been fed. If you receive and occasionally take 3000+ calls over a period of years from some crazy person, they think that you’re in some sort of relationship.

Fragile people act out in unpredictable ways. After 10 years, this isn’t a kid anymore.