A Microsoft file hosting and synchronization service.
I have the same question - though I notice that if you click on "Sign in another way" there is an option to send a request to phone which uses the authenticator app - I'd like this to be the default as it previously was and I also do not want access to be granted using the pin.
"Because you're accessing sensitive info for ******@hotmail.com, we'll send a request to your phone to verify your identity."