I got a call from this number claiming to be Vanguard and asking for my security question in order to settle some "time-sensitive" issue which I dont believe exists for me. For a moment it sounded very legitimate - the accent was right. Fortunately I got cautious and asked the caller how I'd know he was from Vanguard. He said "you are right, there is absolutely no way of knowing it. I will email a number to call back" . I havent received any email. However I am a bit concerned that he knew my full name including the middle name
Got this number today, and it was legit. The man stated, as others here noted, that there was a time sensitive issue, etc. and needed some account info for verification. I refused to give that information over the phone. When I called back the REGULAR customer service number, the issue was about updating my address. I gave them the number that called me, and it was one of their representatives. I asked why Vanguard asked for security info over the phone, and was told that this is their policy. I don't think it's a great policy, but it is what it is.
That being said, I'd not give any info over the phone to a random person - call back on the regular customer service line and handle the issue that way.
Due to caller ID spoofing (faking) you cannot assume the caller is who they say they are. Never give out personal or financial information on a call you did not make. Look the number up. Verify it. Decide from there.