USCIS has issued instructions on making inquires with four Service Center. The step-by-step instructions are as follows:
Step 1: contact the National Customer Service Center at 1-800-375-5283.
Step 2: if more than 30 days have passed since your contacted the NCSC, you can email the proper USCIS Service Center to check the status of your case.
California Service Center: csc-ncsc-followup@dhs.gov
Vermont Service Center: vsc.ncscfollowup@dhs.gov
Nebraska Service Center: ncscfollowup.nsc@dhs.gov
Texas Service Center: tsc.ncscfollowup@dhs.gov
Step 3: in case you do not receive a response within 21 days of contacting the Service Center, you may email the USCIS Headquarters Office of Service Center Operations by email at SCOPSSCATA@dhs.gov.
For more information on USCIS and its program, visit http://www.uscis.gov/.