I recently bought an XBOX 360 Live Gold Membership so I could upgrade my Silver membership to Gold for additional benefits you get.
I bought a pre-paid ticket from Game and the idea is you simply scratch off the back and enter the code and it upgrades your XBOX Live account. After I tried this via the XBOX I received an error code (can't remember the number - should have made a note of it). So before phoning customer support, I tried again but this time via http://www.xbox.com using my laptop but received a different error, something along the lines of "the code was invalid or not for this web site...." After phoning support, support knew straight away it was because I didn't set my address, email and phone number in my profile! After setting this information, it worked a charm.Very good support I may add, would of been useful for a more meaningful error message though or validation.
So if you decide to upgrade your XBOX Live Silver account to Gold in the near future, remember to set your address, email and telephone number in your profile!