Apparently this is no fraud attempt - NatWest are just using sales staff who have no comprehension of what customer service is. (Hanging up in your ear? Seriously!).
Below are the contact details for the NatWest Fraud department. (But don't take my word for it - always check for yourself!)
"If you suspect that you've been the victim of fraud or notice any suspicious transactions on your NatWest account, you can report this to us by telephoning 0845 300 3983 (24 hours a day) or by contacting one of our branches. From outside the United Kingdom, please telephone (0044) 131 523 8383 (24 hours a day). Please note that charges can be reversed on calls from abroad.
If you need to report fraud in relation to a NatWest credit card, you should telephone 0845 300 4350 (24 hours a day). From outside the United Kingdom telephone (0044) 1268 508 020 (24 hours a day). Please note that charges can be reversed on calls from abroad."
http://www.natwest.com/global_options.asp?id=GLOBAL/SECURITY/REPORT_FRAUD