I am not sure which center I called, but our cases got transferred from VSC to TSC, and at the time I made phone call, our cases were at TSC. The following calling procedure worked for me.
Here is how:
Dial: 1-800-375-5283;
press: 1->2->2->6->2 (not 1. 1 is for entering receipt number. don't enter receipt number if you want to talk to representative)->4 (to talk to customer representative).
They may ask you some info. so the information that needs to be handy is (all the info. is on form 797): R#,A#, when and where sent application, name, date of birth, mailing address,email address,phone#,etc.
Good luck.