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Since Tmo can't find your account, can you get them to send something to you in writing (mail or email) to that effect? I had to do something like that to "prove" I didn't owe money, they were kind enough to send an email and I printed it and used that for my disputes (it worked).
Good luck!
You should be able to request something that says you do not have an active account, if the Tmo helpline is of no help, perhaps try going to a local Tmo store to see if you can be provided something by managment at a store level.