I get a phonecall:

Hello, it that Mr Charles [middle name removed for reasons of paranoia] Elwood?

Who's calling?

I'm very sorry, but I can't tell you that until you confirm your identity, are you Mr .....

You've not got the hang of this have you? If you're going to ask me to verbally authenticate myself you're going to have let me know who you are, or who you are calling on behalf of, so I can cross-reference the number.