To be honest even after I had seen that the charges being described to me on the phone were indeed on my account I was still running through scenarios where this could be a fraudulent call. However, I eventually came up with the logic that somebody had already managed to put the charge on my account, and once they'd gotten that far, I couldn't think of any reason for the scammer to call me. Basically, scammers had visibly already won prior to the call, so the odds of the call being fraudulent were low.