I did. As mentioned, I tried everything (well, most everything - admittedly, I didn't think of moving the SIM card to my own phone as the above poster stated. Hindsight. Gotta love it).
The T-Mobile rep wanted me to take the phone in to a store at first but I then explained that I was over an hour away from the nearest T-Mobile store (I live in a rural area). I was then passed to another person who looked up the account. Fortunately, they had a contact point for the owner. I then gave her one of my emails along with my cell number and asked if she could pass it along. As of today the owner hasn't responded. At least I can tell them where to go to pick the phone up, so...good thing. Kinda frustrating, though.