The worst is the "this is an important message about your credit card...please stay on the line to hear how we can reduce your interest rate" scam. I have stayed on long enough a few times to actually speak to someone and they are wanting to 'confirm' your credit card information including the full account number, DOB, and SSN before they can offer you their promotion.
Obviously knowing this is a scam, I let them finish their talk and then I ask who they are affiliated with and am instantly hung up on. When I call the number back it is disconnected. Crafty little fuckers.
-Marc