As it turns out, the Nokia Lumia 1520.3 (RM-938) is a version of the phablet for third parties, and it apparently offers connectivity with the same AWS bands used by...good guess...T-Mobile. In addition, this model also works on AT&T's LTE network.
Since it is unlocked, and not tied down to AT&T's network, this version of the Lumia 1520 uses the Qi standard for wireless charging instead of the PMA standard used by AT&T. In addition, the unlocked version comes with 32GB of native storage, double the amount that the carrier locked AT&T unit offers you.
You can find this model at Negri Electronics and Newegg both only in black. White and yellow variants are back ordered. Pricing is in between $624 and $657, so it is not cheap. But if you are a T-Mobile customer who wants this phone more than anything, this is the price you must pay for your feelings.
source: NegriElectronics, newegg via WPCentral