[ars]

I'm very confused about what this is.

Duo as in Google's Duo video calling? There's 2FA on that? I've never seen any.

Or is there some other Duo it's referring to?

[amanzi]

I assume this is referring to Duo Security, owned by Cisco: https://duo.com/

[evancox100]

I was just as lost as OP. Can someone also explain how it uses DTMF tones? Is it using tones to deliver or receive 2FA codes? Scanned the above url but didn't see the info.

[URSpider94]

You register with this service, get a new phone number from them that they auto-answer, then provide this number to the security service. When the security service calls this number, it will play back DTMF codes to simulate whatever buttons you need to press to enter your code key.

[flaviu1]

It calls you, and then you need to pick up and press the one key to authenticate yourself.