@URH1
Thanks.
That's true, "optional."
For the people who "need" to be found that might be an option. I think that is rather unwise, though.
A phone number is stored in so many webshops, etc., there's hardly a better primary key for aggregating information about a person.
So, hands off the phone number option.
Selling each bit of information there is of users is a multi-bn $ global industry.