In Spain you can't find people by their telephone number (mobile or at home). It's forbidden.
With the Apartado de Correos you can't because this kind of thing is designed specially for people who don't want to let other people know where they live or where they have their bussiness.

The only way to find someone is knowing name and surname, city (sometimes you just need to know province) and only when they have ther home telephone number under their names (not under the name of other person in the house). Some people have phone number at home but they request to maintain ir private so they are not in white pages.

You can try in www.paginasblancas.es. You need one personal data (at least name, if it's unusual) and at least the province.