You need to find your list of phone numbers from somewhere. Presumably, from this question, you want to do lookups against a third-party list. Your best bet would be to call that provider and ask them if they have an API you can use. It might cost you a little bit, but you will at least have legitimacy.

This is especially important if you feel the need to do this fast. Presumably, because of the implied performance aspect, you plan on making money with this venture. If you are reselling this information, you need to make sure that you have cleared your commercial usage of a third-party's information with said third-party.