On the official website of the Department of Citizenship and Immigration Services, the United States Citizenship and Immigration Services Office includes both the phone number for its national customer care line as well as the phone numbers for local offices. The phone number for the national customer care line is (800) 375-5283 as of the year 2015.
According to USCIS.gov, calling the national customer service line is a more practical option for obtaining information than mailing a letter or taking time off work to visit a local office. This is because calling the national customer service line is free of charge. Help is offered, interviews are conducted, and more information is provided in addition to that which is accessible via the national customer service line by both domestic field offices situated inside the United States and international offices located in certain foreign countries.