Over the years, many people who were stuck in the Facebook platform have been coerced by Facebook into providing a phone number to “verify” their account. The other choice given to them was (and is) to lose access to the account because Facebook believes it’s a fake account or a spam account or a “violation of its community policy”.
In other cases, Facebook may get the phone number because someone uploaded their address book/contacts to it. This information shouldn’t be in the user’s public/private profile (even though Facebook would store it internally, use it to figure out other connections and “show relevant ads”).
