I think it was a 10 digit number and letter code which I could never remember and would have to then write down. Granted, this is not Amazon, it was some other company.
And yeah, I forget my phone all the time. To me, the phone is something I use, not something that I need or that others need me to have. On the weekends, I maybe check it twice if I'm not hiking or something, it just sits there charging otherwise.
Who forgets their phone these days? And even then, you can memorize the 4 digit code.
We have these things and they work great.