Regarding this matter, especially when the "banks" call to ask for personal details, my friend have this nice habit of asking which department they're calling from & then insist that she'll call them back shortly.

When she calls back, she'll call the Bank's general line & asked to be forwarded to the particular department. At least this way, you'll be 109% sure that you're talking to the bank. Unless you dailed the wrong general line number

Well, my friend has this habit of insist-to-call-back whenever anyone claiming from the bank call her. This is because, when you are the one making the call to the bank, you are 100% sure that you're talking to your bank (unless you dialed the wrong number).

So in your case, if you're truly interested with the promotion, insist you'll call back the bank directly to sign up or register. If the promotion is real, the customer service hotline will know what to do. If the customer service don't know what you're talking, chances of the earlier call being a scam is high.