Duo is for two factor authentication, where my impression is that this is for signup verification. Consider mobile apps like WhatsApp, which use your existing

Duo is for two factor authentication, where my impression is that this is for signup verification. Consider mobile apps like WhatsApp, which use your existing mobile number as your identifier. This makes a lot of sense for a bunch of reasons, but it requires the app to somehow verify that the number you enter is actually your number. This is commonly done by having the service send a verification code in an SMS, which you then enter back into the app.