Automatically singin with a mobilephone's phone No

Maybe this is an odd question an probably not at all possible. But i try anyway.
When a user logs on with his mobile phone:
Would it be possible to detect  somehow the user phone no, and use it as an automatically sign-in to a web-portal.
Or if this is not possible is there a way to grabb a kind of unique key from a pc or mobile phone which i could use as a automatic logon procedure..
Any help would be appreciated..
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