how did you know it was them then?!

@Oz taha I googled the number and called them back, it was my local field office

For me, employment verification. My FO at San Diego, he attempted calling 2 times then he texted so I called him back r.a. Subsequent attempts won't get answered for my case. Hope it helps.

@T P Thank you for sharing - had you changed job when they called you for employment verification? And what was the outcome after that?

No job changed. I can only think of, it was one of the last routine checks before decision. I'm still waiting. Currently at RFE Response.