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.