Whenever you get a receipt number from USCIS for H1b application or Green card application or any related application, USCIS sends an acknowledgement after receiving the application. It also assigns a receipt number before processing the case. This number contains 3 alphabets followed by numbers. The first 3 letters indicates the service center your application is going to be processed at. This indicates as follows:
EAC -- Vermont Service Center
SRC -- Texas Service Center
LIN -- Nebraska Service Center
WAC -- California Service Center
Very recently USCIS streamlined the process for case inquiry on any application submitted.
USCIS wants the applicants to call the customer service center and tell the issue you are calling about. Then, they want you to wait for 30 days, if the issue is not resolved, they ask you to email the corresponding service center as per your application. Then, they want you to wait for 21 days. If there is still no response, they want you to mail the USCIS headquarters. At this point, they guarantee you will get a response within 10 days of your email.
Complete information of the phone numbers to call and email address to email can be found at the link here : USCIS website link