Most of those sites have to pay per number that is searched. If I remember correctly theyhave to contact directlly the carrier.

I had a friend a few years ago who wrote a program that would let people do a search on his site, and the program would retrive the information from the site he was paying for, so people could do it for free (he mad his money back with advertisments). But the company found out and was not very happy

See More: reverse cell phone search