Not to drag this out, but...
There were no pre-paid phones in the T-Mobile location I visited.
The "Team Member" at Target told me that the pre-paid phones don't work with regular plans. He might have been referring to other carriers, but he left me with enough doubt that I walked out empty handed.
This is why I went to an official T-Mobile store in the first place. I realize there's little profit to be made by selling a flip-phone with no data plan, but if you no longer want the steady income stream from the voice/text-only customers, why not just say so?