Web API two factor token authentication

Hi,

I am trying to implement a two factor authentication for an angularjs web application that uses Web API 2 as backend.

Authentication is currently done by requesting a authentication token from Web API by passing username and password.

Authentication token is generated by ASP.NET Identity and it is an oauth bearer authentication token.

How do I modify my current username/password authentication so that it enables two factor authentication by for example sending an SMS message to the users cell phone?

Thank you,

Matti Petrelius