I got this voicemail the other day from "Rachel at cardholder services":
(If the embedded player doesn't work, here's the direct link)
This one is kinda clever, that rather than a human using high-pressure tactics to get you to enter your credit card number, what you hear instead is a recorded message which asks you to "press 1 to get a lower interest rate". Had I pressed 1, I suspect I'd be transferred to a nice sounding human operator who would try to coax me into giving them my credit card number.
There's two takeaways from this:
1) Never give out your card card number to someone who calls you on the phone. (caller ID can be spoofed)
2) Strongly consider against picking up the phone when an unknown number calls you. Let it go to voicemail. If it's someone trying to get a hold of you, you can listen to the voicemail right away (or use Google Voice, which does transcripts), and call the person back.