Contact the National Customer Service Center for the U.S. Citizenship and Immigration (USCIS) office by calling 800-375-5283, as of June 2015. The contact number for the hearing impaired is 800-767-1833, as stated on the USCIS website.
The USCIS website is filled with different contact numbers and addresses. To contact the military helpline, call toll-free, 877-247-4645, and adoptive parents who need to inquire about a Hague adoption can call 877-424-8374. Employers who want to verify the citizenship of a prospective employee can call E-Verify at 888-464-4218. These numbers are valid as of June 2015 and are subject to change without notice.
The USCIS, run by the Department of Homeland Security, offers email options for users who prefer this form of communication. Details can be found on the website.