What kind of numbnuts trusts a phone number given to them by the person being authenticated? "Here, call my accomplice-- er, I mean account rep, and he'll verify me." Yeah, pull the other one.
Unfortunately, even my credit card issuer can't get this right. They called me about some charges. "Now sir, to verify that I'm really talking to the account holder, what is your social security number?" Um, no. YOU called ME. You can reasonably assume that the phone number you have on file for me is valid. It's up to YOU to prove to ME that you're from my credit card company. "But sir, we ask this for your own security..." Eventually I got them to give me a ticket number so I could call the number printed on my card and get back to them. Turns out it actually was my issuer calling, not a scammer. Guys, you really should know better!