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.
https://www.vice.com/en/article/y3g8wb/hacker-got-my-texts-16-dollars-sakari-netnumber
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.
