PhoneVal API

From their site: Our PHONEval Web Service is your fist defense against acquiring and storing inaccurate data. This service validates the NPA and the NXX (area code and prefix) of all the North American Dialing Plan range of phone numbers. In addition to verifying the validity of the phone number, this service will also return each zip code where this NPA/NXX combination exists. It will also give you the number of active phone numbers in each of these zip codes. We update our databases every 30 days to ensure we provide the most accurate data available.