One of the problems of US phone numbers is that you can't tell if they are fixed or mobile. If you registered a (roaming) mobile number then obviously the process would work. I have had (the HK versions of) Citi, Amex and HSBC all call me successfully regarding transactions when abroad. The cost of the roaming call is a bit annoying, but if they don't do it too often then I appreciate the feeling of extra security for my account.