That sounds very fishy, since its a gov agency idk why they would call. Anything to do with them is usually put in writing. When I spoke to a level 2 officer regarding my application, they sent me a letter for my record. I would email the service center that is processing your case directly and explain this situation. Also include the phone number you got the call from, date & time you received the call and the fax number that was given to you. The call could be legit, but you have to play safe because this is a one time shot of either getting approved or denied