Android :: IPhone Or Android Apps That Use SMS Based Authentication?

What are some iPhone or Android applications that use SMS as their primary means of user authentication?I'm interested to see such apps in action. SMS-auth seems like a natural approach that is well-situated to mobile contexts.

The basic workflow is: to sign up,a user provides a phone number; the app calls a backend webservice which generates a signed URL and sends it to the phone number via an SMS gateway; the user receives the SMS, clicks the link, and is thus verified and logged in. This results in a very strong user identity that is difficult to spoof yet fairly easy. It can be paired with a username or additional account attributes as needed for the product requirements.Despite the advantages, this does not seem to be in much use - hence my question. My initial assumption is that this is because products and users are wary of asking for / providing phone numbers, which users consider sensitive information. That said, I hope this becomes an increasingly more commonplace approach.