In other words, if a company tells you they need your number for account security, refuse to give it to them. Of course you can give out a phone number for general contact purposes.
I think it's easier to teach people phone numbers can't be used for security, period, then to teach them that phone numbers can be used for security in one very specific case -- especially since companies like Facebook/Twitter rarely use terms like 2FA when asking users for their number. They just say stuff like, "you'll use this to gain access to your account" or, "we'll use this to help keep your account secure."
It wasn't my intention to make it sound like users should never tell anyone their number for any reason, but I can definitely see how it might read that way.
What is this supposed to mean? What is the point of having a phone number if you never give it out?
I suppose you can make outbound calls only with own number sending turned off, but that sort of diminishes the purpose of owning a phone number?