Use mobile phone number as account ID so users dont have to register? Im making a mobile first site. Is there a way of using a user's mobile phone number so they dont have to register for an account?

Or, if a user creates an account the normal way on a desktop and enters their phone number, is it possible to detect when the visit the site with a phone from that number and log them in?

Thanks
If I get you right you want detect that a user is recuring user? Generaly this can be done with a cookie.

Tecnicaly there is no reason to use a phone number. As far as I know there's no possibility for a web application to get the phonenumber automatically. A solution could be an app as frontend which delegates to the website.