serial number

was it purchased from TMO or from elsewhere and brought over to TMO from another carrier?

if you purchased it from TMO you can contact them either by call or through Facebook/Twitter and they should have the info you need. if you brought it in as a BYOD and the correct info was never updated then they more than likely wont have it.

do you by chance still have the box for your phone? it would be on that as well.