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Each line needs to have it's own login and password for verizonwireless.com.  Once logged in with the individual phone number, you can set up the ringback tones (and other features) for that specific line/phone number.

Go to verizonwireless.com and REGISTER each number for it's own account.  Only the primary will see all the lines' info; the secondary accounts will see only the shared data and their own details.  If you are getting that the account is already registered, you need to completely log out, clear cache and cookies on the browser, and try again, or use a different browser to register the account.