It's a rather recent thing, and it's touted as a security feature. To be fair, Japan seems to be safer than most of the world when it SIM swapping (there are pr

It's a rather recent thing, and it's touted as a security feature. To be fair, Japan seems to be safer than most of the world when it SIM swapping (there are pretty strict identification requirements defined by law) and for porting phone numbers between SIM cards. I also never heard of any case of attacks against SS7 or other part of the telecommunication stack in Japan (I'd be happy to know if someone does) That being said, a certain class of SIM cards (SMS-only cards, without voice functionality) is exempt from most of these strict checks as far as I know, and there are other technical vulnerabilities that are probably just waiting to happen in Japan before they're taking seriously. I'm a little bit surprised that Yahoo! Japan went for SMS as the only authentication method, since their one of the main sponsors of the FIDO Japan WG.