Almost invariably there's a teeny tiny error so that the email addresses for the two accounts are _almost_ the same. Please go to http://xbox.com/accounts and sign in with the email, you will see its matching Gamertag in orange at upper right.
Then, on the other gamertag, go to the Xbox, sign in with it, then go tap on the silver Guide button in the middle of the game controller. Go two clicks right to Account Management and choose that, then go to Account Security, pick that, and select Change Windows Live ID; the Windows Live ID for THAT gamertag will appear on the right side.
This will save you a call > 99% of the time.