For context, Japanese mobile lines for children include net filtering by default. Also, mobile lines require positive ID to get, so you can't simply get a burner mobile line. A phone number that can send or receive phone calls or SMS is tied to a real world identity, and transferring is illegal.

So, a neat way of requiring ID checks is to simply offload these things to carriers with phone number validation.

In contrast, the US is an example of a country where getting a phone number with no stringent ID check is trivial and it can easily do SMS and phone calls without a second glance.

*ID verification was recently tightened to require reading the IC card data from My Number Card (the national identity card) or in-person KYC for non-IC card users (like a copy of family registry).