T-Mobile: Sir, you'll have to go to the store where the charges were incurred and dispute them.
Ovid: How can I do that if you can't tell me which store this was?
T-Mobile: Hmm, I guess that's a problem.
After digging in deeper, they agree that there may be fraudulent activity on the account. Seems they were unable to tell me how I upgraded my phone seven times in the same day at the same store!
They say they'll call back and this time they promise they mean it.