Its amazing the telecommunications industry still uses or relies on "Letter of Authorization". Its less secure than faxing a piece of paper on "letterhead."
I've said this before, check the archives.
While adding a number, Sakari provides the Letter of Authorization for the user to sign. Sakari's LOA says that the user should not conduct any unlawful, harassing, or inappropriate behaviour with the text messaging service and phone number.
But as Lucky225 showed, a user can just sign up with someone else's number and receive their text messages instead.