Discussion on answer by Alexander The 1st: Why is SMS used as a way of verifying a user's mobile, when it is not even encrypted in transit?

Imported from a comment discussion on https://security.stackexchange.com/questions/252076/why-is-sms-used-as-a-way-of-verifying-a-users-mobile-when-it-is-not-even-encry/252100#252100