I have had a couple of these calls. Called back on a non identifiable line assuming it was a scam call. I had to let the phone ring a long time (10 rings) and the ring was very strange (broken ring) as if it was being routed to another place. A woman answered and there was lots of noise in the background as if she was at a pay phone or sitting in a call center with lots of people talking. I didn't talk to her but she identified herself as IRS.