If this person is a Spanish national you'll have more luck. If the phone number's in his/her name you'll have MUCH more luck. If you know BOTH LAST NAMES of this person (and correctly spelled), even more luck. But if the person is a minor or married woman you aren't likely to have any luck.
There was a recent thread here in this "Classifieds & Personals" forum category from someone like yourself searching for someone they once knew who lived in Madrid. It's a tough search unless the person you're seeking is an adult male and more than 40 years old.
Check Spain's white pages @ http://www.PaginasBlancas.es
Good luck! Saludos, MadridMan
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