Well, you did mention google as example, and while it is technically "opt-in" I guess, since they haven't disabled my account yet, they ask every third log-in whether I don't want to enter my cell phone number for security, and then I click no, and then they ask again, whether I'm really sure that I don't want to enter my cell phone number, and then I have to click no, and that starts all over again every few days.