Facebook 2FA numbers are also shared with Instagram which prompts you 'is this your phone number?' once you add to FB.
That is why every startup wants your phone number.
It's shocking that this one number is used for usernames, authentication (2FA), advertising tracking, geolocation and more. And it's the same piece of info you have to give to a random plumber to come and fix the boiler.
Delete a Facebook or WhatsApp account after this integration and they'll keep your data, claiming it's used or needed for Instagram. The trend line couldn't be clearer. nytimes.com/2019/01/25/tec…
TL;DR: Login-with-Phone-Number is the new Login-with-Facebook. Easy to track, shared between services, it's the key to invisible mesh of your data. Don't do it.
Each time a service requires a phone number, iOS could generate a new number. If Apple is serious about user privacy, this is the next frontier. The current one, really.