I wouldn't call them, just sign up! I started signing up with my UK address, but when I came to the end of the form I discovered you had to state a US state and ZIP. So I just picked the one I had when I studied in the US ten years ago. I thought they might catch me out by comparing the address with the ZIP, or even with the address of my credit cards, but they haven't and I've had miles post to my FF account. You may want to ensure that the state and the ZIP that you provide correspond, however, just in case...