When I had a Twitter account, I had to provide a fake number to continue using it, but I was active in different political discussions and that might be the reason. Twitter didn't explain why it required a phone number, except for saying it was necessary for security or something like this.

Also, Microsoft will require a phone number once you try to log into an Outlook email account from other IP address than you have signed up with. Again, it says the number is necessary to "secure an account from hackers" or something like this.

> Also, Microsoft will require a phone number once you try to log into an Outlook email account from other IP address than you have signed up with.

That's quite a good idea.. They're effectively using your IP as a 2nd factor auth for those people who refuse to use 2 factor. If, like many people, you have a static IP at home, and they whitelist IP's your sessions roam to, you may never need to log in from a new IP.